libroot: Replace most of libm with musl's.

The glibc libm code was showing its age, and has recently been
the subject of a number of tickets about its inaccuracy.
Additionally, some developers have complained about
how convoluted the headers are, and thus how hard it is
to add support for new architectures (and how flaky
the support for the existing architectures is.)

So, with this commit, nearly the entire glibc libm has been
gutted and replaced with the one from musl 1.1.24.

The complex functions from glibc are retained (as they
are more mature than musl's), as are some glibc-internal
libm functions.

This also has the advantage that these functions are
actually using our <math.h>, whereas GCC used its own,
which was rather dangerous for obvious reasons.

Additionally, the new math functions are always compiled
with GCC 8 (even on x86_gcc2), as it seems GCC 2 does
not quite understand some of the union-aliasing they
use (a lot of which was added in C99, I suppose.)
FFmpeg on x86_gcc2 is already compiled with GCC 8
and that has so far worked out well, so there should
not be any problems caused by this.

I did verify that ARM and PPC at least still compile,
though other architectures may require a bit more work
(they are not bootstrapped so I could not do much.)

Should fix #14933 among other issues.

Change-Id: Ifeea0ddab23a8d0480fc26dece1b0192afc263bd

sparc: use generic w_sqrtl.c implementation.

I accidentally imported a newer version of the file that does the same
thing as the existing one.

Change-Id: Ifab3b24218f017cc57a459e0467cb8225accd9e5
Reviewed-on: https://review.haiku-os.org/c/1368
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

glibc: cleanup long double support

- Add 128 bit long double support from current glibc and a few headers
they need
- Move the existing 80 bit float support in a sub directory of generic,
it is not universal to all archs (see file added in docs/develop/arch).
Also include some new .h files for x86 that are needed after these
changes (from newer versions of the glibc).
- Adjust Jamfiles for m68k, x86 and x86_64 to use the 80bit format
- Do not adjust arm jamfiles, it was wrongly using 80bit long double and
should be fixed to use 64bit instead (which means the double functions
can be used with aliases)
- Do not adjust powerpc jamfiles, because it uses yet another format and
we build it without long double support anyways.

Note that I moved only the files that were creating compile errors,
quite likely more of the s_* and e_* files need to be moved to the
specific directories, see glibc list here:

https://sourceware.org/git/?p=glibc.git;a=tree;f=sysdeps/ieee754/ldbl-128
https://sourceware.org/git/?p=glibc.git;a=tree;f=sysdeps/ieee754/ldbl-96

Change-Id: Ic2d8a454ba9a3b99638e4fbb63daf02df0fea403
Reviewed-on: https://review.haiku-os.org/c/1143
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

sparc: get libroot to build

Add various stubs to fix undefined references. No implementation for
anything yet.

Change-Id: I2d398bc2369d099e3a35f0713058d6a5edc6801d
Reviewed-on: https://review.haiku-os.org/c/1138
Reviewed-by: waddlesplash <waddlesplash@gmail.com>