Log Message:
Darwin: Fix an issue that gcc producing binaries linked with a wrong libgcc_s

If we don't install libgcc_s.10.[45].dylib, our gcc links binaries
with *both* /usr/lib/libgcc_s.1.dylib and
${GCC_PREFIX}/lib/libgcc_s.1.dylib, which is certainly a bad thing.

The problem was already reported to the upstream but it caught
seemingly no attention:
http://gcc.gnu.org/ml/gcc-help/2010-07/msg00164.html