[libc++][CMake] Allow merging libc++abi.a into libc++ even on Apple platforms

Summary:
I can't see a good reason to disallow this, even though it isn't the
standard way we build libc++ for Apple platforms.

Making this work on Apple platforms requires using different flags for
--whole-archive and removing the -D flag when running `ar` to merge
archives because that flag isn't supported by the `ar` shipped on Apple
platforms. This shouldn't be an issue since the -D option appears to be
enabled by default in GNU `ar`.

Reviewers: phosek, EricWF, serge-sans-paille

Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59513

llvm-svn: 356903