sparc: add arch-specific libroot stubs

These are helper functions for long double math. On sparc there is no
hardware implementation of long double instructions, and the ABI defines
these methods which must be part of the C library. GCC calls the methods
directly in its generated code.

They were already added to libroot, but they also need to be in the
stub, which is used during bootstrapping to link mpfr (during the
target gcc build).

I put these in a separate file because I assume the generation of the
stubs file from the real libroot.so will usually not be run on sparc,
and will not include these symbols otherwise. This may become a problem
if libroot for different architectures starts to diverge further.

Change-Id: I6070394c685fee35b3dc12a507f5a6271571b993
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4636
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>