[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@362311 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed