[lld-macho] Make `--icf=safe` work with LTO

Just matter of enabling the config option.

(Also changed the platform of the input test file to macOS, since that's
the default that we specify in the `%lld` substitution. The conflict was
causing errors when linking with LTO.)

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D127600