Remove sparc64 specific buid-system hacks
Remove all the sparc64 hacks and tweaks to the build system. We don't
need them anymore.
Remove sparc64 specific parts of rtld-elf.
Remove sparc64 specific parts of libm and fix comments
Once upon a time, sparc64 was the only ld128 architecture. However,
both aarch64 and riscv are now such architectures. Many of the
comments about how slow multiplication was on old sparc64 processors
are now no longer true. However, since no evaluation has been done for
aarch64 yet, it's unclear if they are still relevant or not. If not,
the code should be changed. If so, the comments should remove the
uncertainty.
Remove sparc64 specific parts of libc.
Also update comments for which architectures use 128 bit long doubles,
as appropriate.
The softfloat specialization routines weren't updated since they
appear to be from an upstream source which we may want to update in
the future to get a more favorable license.