Allow building both shared and static library

This change allows building both shared and static version of libc++
in a single build, sharing object files between both versions.

Differential Revision: https://reviews.llvm.org/D23232

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@278068 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed