[cmake] Add custom command to touch archives on Darwin so ninja won't rebuild them.

Summary:
clang and newer versions of ninja use high-resolutions timestamps, but
older versions of libtool on Darwin don't, so the archive will often
get an older timestamp than the last object that was added or updated.
To fix this, we add a custom command to touch the archive after it's
been built so that ninja won't rebuild it unnecessarily the next time
it's run.

Reviewed By: beanz

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62172

llvm-svn: 361280