[libc++] Add support for cross compiling.

Reviewers: EricWF, jroelofs

Reviewed By: jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6990

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@226237 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed