-gc-sections is called -dead_strip in Apple's linker

[Roger-Shepherd]

The files support/dummy-libgcc.c, support/dummy-libc.c, support/dummy-libm.c refer to -gc-sections, which is an option to the (gnu?) linker to remove unused code. The Apple linker uses -dead_strip for apparently the same purpose. Perhaps the dummy-lib*.c's should be worded to accommodate this.

The usage of -gc-sections in config/native/* may be OK, depending on what "native" is supposed to do.