[cmake] Add linker detection for Apple platforms

LLVM currently assumes that Apple platforms will always use ld64. In the
future, LLD Mach-O might also be supported, so add the beginnings of
linker detection support. ld64 is currently the only detected linker,
since `ld64.lld -v` doesn't yield any useful version output, but we can
add that detection later, and in the meantime it's still useful to have
the ld64 identification.

Switch clang's order file check to use this new detection rather than
just checking for the presence of an ld64 executable.

Differential Revision: https://reviews.llvm.org/D48201

llvm-svn: 334780