[libunwind] Remove checks for -nostdlib++ (#143162)

libunwind uses a C linker, so it's never even trying to link against any
C++ libraries. This removes the code which tries to drop C++ libraries,
which makes the CMake configuration simpler and allows for upgrading
GCC.

GitOrigin-RevId: c59cc2b690b9e528a82ba214f74a8f7c8abb3cde