[Darwin] Introduce a new flag, -fapple-link-rtlib that forces linking of the builtins library.

This driver flag is useful when users want to link against the compiler's
builtins, but nothing else, and so use flags like -nostdlib.

Darwin can't use -nolibc & nostdlib++ like other platforms on because we
disable all runtime lib linking with -static, which we still want to have
an option to link with the builtins.

Differential Revision: https://reviews.llvm.org/D58320

llvm-svn: 360483