Android.rules: Add libc++ support

Summary:
This adds support for building libc++ tests when targetting android. The
tests are still not passing due to several other problems, but this way
we can at least build them.

Reviewers: eugene, EricWF, danalbert

Subscribers: srhines, lldb-commits

Differential Revision: https://reviews.llvm.org/D30737

llvm-svn: 297616