FirstBootPrompt: Exiting is now possible

- Exiting the window when the Desktop is not running will result in a reboot.
- Otherwise, the window will just close ordinarily.
- A confirmation box pops up before rebooting.
- Made other optimizations for running FirstBootPrompt on the Desktop.
- Moved some definitions to the header files.
- Added "UsePrivateSystemHeaders" to the Jamfile.

(Thank you, PulkoMandy!)

Change-Id: I34d6538d7b538d36448498ad1ecd442d76bd2bb6
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3204
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

Firstbootprompt: Localize keymap list

Fixes #15522

Change-Id: I8909f70c0f7abe1fa23ea7c145220b95a3e06395
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1718
Reviewed-by: Niels Sascha Reedijk <niels.reedijk@gmail.com>

Build: Add architecture rule for libshared.a.

- As suggested by Ingo, add libshared.a to the architecture name map.
This allows it to be linked by its short name like other frequently
used libraries.
- Adjust all Jamfiles referencing the lib accordingly.

FirstBootPrompt: launch installer/desktop targets directly.

* No need for shell scripting here.

Use libstdc++, libsupc++ and libgcc from gcc_syslibs.

* Instead of faking libstdc++.so from libstdc++.a, use libstdc++.so
from the gcc_syslibs build feature for everything except x86_gcc2.
* Use libgcc_s.so from the gcc_syslibs build feature for everything but
x86_gcc2 (which still carries libgcc as part of libroot.so).
* Drop filtering of libgcc objects for libroot, as that is no longer
necessary since we're only using libgcc-as-single-object for libroot
with x86_gcc2, where the filtered object file doesn't exist. Should
the objects that used to be filtered cause any problems as part of
libgcc_s.so, we can always filter them as part of the gcc build.
* Use libsupc++.so from the gcc_syslibs build feature for everything but
x86_gcc2.
* Adjust all Jamfiles accordingly.
* Deactivate building of faked libstdc++.so for non-x86-gcc2. For
x86_gcc2, we still build libstdc++.so from the sources in the Haiku
source tree as part of the Haiku build .
* Put gcc_syslibs package onto the image, when needed.

More work towards hybrid support

* All packaging architecture dependent variables do now have a
respective suffix and are set up for each configured packaging
architecture, save for the kernel and boot loader variables, which
are still only set up for the primary architecture.
For convenience TARGET_PACKAGING_ARCH, TARGET_ARCH, TARGET_LIBSUPC++,
and TARGET_LIBSTDC++ are set to the respective values for the primary
packaging architecture by default.
* Introduce a set of MultiArch* rules to help with building targets for
multiple packaging architectures. Generally the respective targets are
(additionally) gristed with the packaging architecture. For libraries
the additional grist is usually omitted for the primary architecture
(e.g. libroot.so and <x86>libroot.so for x86_gcc2/x86 hybrid), so that
Jamfiles for targets built only for the primary architecture don't
need to be changed.
* Add multi-arch build support for all targets needed for the stage 1
cross devel package as well as for libbe (untested).

Renamed ReadOnlyBootPrompt to FirstBootPrompt.

The previous name is no longer applicable, as *BootPrompt is now launched on
the first boot of writable medium (e.g., USB thumb drive and hard drives).
Fixes part of #9399.

Notes:
* BootPrompt.rdef may have lost its history.
* The renaming was done with the following command:
git mv src/apps/readonlybootprompt src/apps/firstbootprompt

FirstBootPrompt: launch installer/desktop targets directly.

* No need for shell scripting here.

Use libstdc++, libsupc++ and libgcc from gcc_syslibs.

* Instead of faking libstdc++.so from libstdc++.a, use libstdc++.so
from the gcc_syslibs build feature for everything except x86_gcc2.
* Use libgcc_s.so from the gcc_syslibs build feature for everything but
x86_gcc2 (which still carries libgcc as part of libroot.so).
* Drop filtering of libgcc objects for libroot, as that is no longer
necessary since we're only using libgcc-as-single-object for libroot
with x86_gcc2, where the filtered object file doesn't exist. Should
the objects that used to be filtered cause any problems as part of
libgcc_s.so, we can always filter them as part of the gcc build.
* Use libsupc++.so from the gcc_syslibs build feature for everything but
x86_gcc2.
* Adjust all Jamfiles accordingly.
* Deactivate building of faked libstdc++.so for non-x86-gcc2. For
x86_gcc2, we still build libstdc++.so from the sources in the Haiku
source tree as part of the Haiku build .
* Put gcc_syslibs package onto the image, when needed.

More work towards hybrid support

* All packaging architecture dependent variables do now have a
respective suffix and are set up for each configured packaging
architecture, save for the kernel and boot loader variables, which
are still only set up for the primary architecture.
For convenience TARGET_PACKAGING_ARCH, TARGET_ARCH, TARGET_LIBSUPC++,
and TARGET_LIBSTDC++ are set to the respective values for the primary
packaging architecture by default.
* Introduce a set of MultiArch* rules to help with building targets for
multiple packaging architectures. Generally the respective targets are
(additionally) gristed with the packaging architecture. For libraries
the additional grist is usually omitted for the primary architecture
(e.g. libroot.so and <x86>libroot.so for x86_gcc2/x86 hybrid), so that
Jamfiles for targets built only for the primary architecture don't
need to be changed.
* Add multi-arch build support for all targets needed for the stage 1
cross devel package as well as for libbe (untested).

Renamed ReadOnlyBootPrompt to FirstBootPrompt.

The previous name is no longer applicable, as *BootPrompt is now launched on
the first boot of writable medium (e.g., USB thumb drive and hard drives).
Fixes part of #9399.

Notes:
* BootPrompt.rdef may have lost its history.
* The renaming was done with the following command:
git mv src/apps/readonlybootprompt src/apps/firstbootprompt