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

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

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