[libc++abi] Add install-cxxabi-stripped target

LLVM is gaining install-*-stripped targets to perform stripped installs,
and in order for this to be useful for install-distribution, all
potential distribution components should have stripped installation
targets. LLVM has a function to create these install targets, but since
we can't use LLVM CMake functions in libc++abi, let's do it manually.

Differential Revision: https://reviews.llvm.org/D40681

llvm-svn: 319499