[libcxx] Use libtool when merging archives on Apple platforms

ar doesn't produce the correct results when used for linking static
archives on Apple platforms, so instead use libtool -static which is
the official way to build static archives on those platforms.

Differential Revision: https://reviews.llvm.org/D62770

llvm-svn: 362311
GitOrigin-RevId: 737de4d363ede4b90dd5609af0494fb39af53865
1 file changed