The standard bsd.lib.mk rule is suitable for libgcc_s's C++ source.
The local rule had the following non-functional argument differences
or additions:
1. -DSHARED (rather than -DPIC from bsd.lib.mk)
The C++ sources don't have an #ifdef for either one.
2. -fexceptions
This is enabled by default for C++ so does not need to be set
explicitly.
3. -D__GLIBC__=3
Not used by LLVM libunwind.
4. -DElfW=__ElfN
LLVM libunwind provides its own definition.
PR: 206381
Differential Revision: The FreeBSD Foundation
c4601d6dfb