[libc++] Create install-stripped targets

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++, let's do it manually.

Differential Revision: https://reviews.llvm.org/D40680

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@319959 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed