Remove local override for .cpp.o and .cpp.po rules

The local build rule used to set -fvisibility=hidden and -fPIC, in
addition to -fexceptions and -D defines that had no effect.

With -fvisibility=hidden and -fPIC in STATIC_CXXFLAGS the standard
bsd.lib.mk rules are suitable for libgcc_s's C++ source.

PR: 206381
Sponsored by: The FreeBSD Foundation