Remove local override for .cpp.So rule

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. -DGLIBC=3
   
   Not used by LLVM libunwind.

4. -DElfW=__ElfN
   
   LLVM libunwind provides its own definition.

PR: 206381
Differential Revision: The FreeBSD Foundation