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>

Fix missing symbols in <stubbed>libroot.so.

* A couple of symbols reuse the same address (via weak symbols), so
we adjust generate_stubs.py accordingly.

Add support for building <stubbed>libroot.so.

* <stubbed>libroot.so is a shared library which contains all the symbols
from libroot, but without any code. This library will be required by
the (to be introduced) stage0 of the bootstrap process, in order to
be able to link the shared gcc syslibs (libstdc++.so, libsupc++.so
and libgcc_s.so).

Fix missing symbols in <stubbed>libroot.so.

* A couple of symbols reuse the same address (via weak symbols), so
we adjust generate_stubs.py accordingly.

Add support for building <stubbed>libroot.so.

* <stubbed>libroot.so is a shared library which contains all the symbols
from libroot, but without any code. This library will be required by
the (to be introduced) stage0 of the bootstrap process, in order to
be able to link the shared gcc syslibs (libstdc++.so, libsupc++.so
and libgcc_s.so).