kernel/x86_64: adjust descriptors tables for compatibility mode.

* also adjust BOOT_GDT_SEGMENT_COUNT for x86, the definition is used by the
boot loader.
* add some 32-bit definitions.
* add a UserTLSDescriptor class, this will be used by 32-bit threads.

Change-Id: I5b1d978969a1ce97091a16c9ec2ad7c0ca831656

haiku_loader: Fix wrong size of gBootGDT on x86_64.

The BOOT_GDT_SEGMENT_COUNT was based on USER_DATA_SEGMENT on both
x86 and x86_64. However, on x86_64 the order of the segments is
different, leading to a too small gBootGDT array. Move the define to
the arch specific headers so they can be setup correctly in either case.
Also add a STATIC_ASSERT() to check that the descriptors fit into the
array.

Pointed out by CID 1210898.

kernel/x86_64: rework of IDT handling code

Similarly to previous patch regarding GDT this is mostly a rewrite of
IDT handling code from C to C++. Thanks to constexpr IDT is now entirely
generated at compile-time.

kernel/x86_64: x86_64 gdt handling code overhaul

Virtually no functional change, just rewriting the code from
"C in *.cpp files" to C++. Use of constexpr may be advantageous but
that code is not performance critical anyway.

x86[_64]: Separate bootloader and kernel GDT and IDT logic

From now on bootloader sets up its own minimal valid GDT and IDT. Then
the kernel replaces them with its own tables.

x86: Let each CPU have its own GDT

Implemented threading for x86_64.

* Thread creation and switching is working fine, however threads do not yet
get interrupted because I've not implemented hardware interrupt handling
yet (I'll do that next).
* I've made some changes to struct iframe: I've removed the e/r prefixes
from the member names for both 32/64, so now they're just named ip, ax,
bp, etc. This makes it easier to write code that works with both 32/64
without having to deal with different iframe member names.

Added arch_cpu.cpp to the x86_64 build.

* Some things are currently ifndef'd out completely for x86_64 because
they aren't implemented, there's a few other ifdef's to handle x86_64
differences but most of the code works unchanged.
* Renamed some i386_* functions to x86_*.
* Added a temporary method for setting the current thread on x86_64
(a global variable, not SMP safe). This will be changed to be done
via the GS segment but I've not implemented that yet.

Merged x86_64 headers into x86 headers.

Not many changes seeing as there's not much x86_64 stuff done yet. Small
differences are handled with ifdefs, large differences (descriptors.h,
struct iframe) have separate headers under arch/x86/32 and arch/x86/64.

haiku_loader: Fix wrong size of gBootGDT on x86_64.

The BOOT_GDT_SEGMENT_COUNT was based on USER_DATA_SEGMENT on both
x86 and x86_64. However, on x86_64 the order of the segments is
different, leading to a too small gBootGDT array. Move the define to
the arch specific headers so they can be setup correctly in either case.
Also add a STATIC_ASSERT() to check that the descriptors fit into the
array.

Pointed out by CID 1210898.

kernel/x86_64: rework of IDT handling code

Similarly to previous patch regarding GDT this is mostly a rewrite of
IDT handling code from C to C++. Thanks to constexpr IDT is now entirely
generated at compile-time.

kernel/x86_64: x86_64 gdt handling code overhaul

Virtually no functional change, just rewriting the code from
"C in *.cpp files" to C++. Use of constexpr may be advantageous but
that code is not performance critical anyway.

x86[_64]: Separate bootloader and kernel GDT and IDT logic

From now on bootloader sets up its own minimal valid GDT and IDT. Then
the kernel replaces them with its own tables.

x86: Let each CPU have its own GDT

Implemented threading for x86_64.

* Thread creation and switching is working fine, however threads do not yet
get interrupted because I've not implemented hardware interrupt handling
yet (I'll do that next).
* I've made some changes to struct iframe: I've removed the e/r prefixes
from the member names for both 32/64, so now they're just named ip, ax,
bp, etc. This makes it easier to write code that works with both 32/64
without having to deal with different iframe member names.

Added arch_cpu.cpp to the x86_64 build.

* Some things are currently ifndef'd out completely for x86_64 because
they aren't implemented, there's a few other ifdef's to handle x86_64
differences but most of the code works unchanged.
* Renamed some i386_* functions to x86_*.
* Added a temporary method for setting the current thread on x86_64
(a global variable, not SMP safe). This will be changed to be done
via the GS segment but I've not implemented that yet.

Merged x86_64 headers into x86 headers.

Not many changes seeing as there's not much x86_64 stuff done yet. Small
differences are handled with ifdefs, large differences (descriptors.h,
struct iframe) have separate headers under arch/x86/32 and arch/x86/64.