libgcc, Darwin: No early install for the compatibility libgcc_s.1.dylib.
On Darwin, GCC now uses a libgcc_s.1.1 for builtins and forwards the system
unwinder. We do, however, build a backwards compatibility libgcc_s.1.dylib.
However, this is not needed by GCC and can cause incorrect operation when
DYLD_LIBRARY_PATH is in use.
Since we do not need or use it during the build, the solution is to skip the
installation into the $build/gcc directory.