[libc++] Merge the Apple install-libcxx and libcxxabi scripts

Also, refactor the now-merged script to remove code duplication in the
creation of universal dylibs.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b68bf11efc46b0a81b22089d6bd78ee9d30a1d22
3 files changed