[compiler-rt] Add install-*-stripped targets

These targets strip during installation, and are required to support
install-distribution-stripped in LLVM (to support a stripped
distribution). LLVM has an add_llvm_install_targets function for this
purpose, but we can't rely on LLVM being present.

Differential Revision: https://reviews.llvm.org/D40687

llvm-svn: 319569