Enable cross-compilation across architectures on android

Summary:
This patch fixes cross-architecture compilation,
by allowing flags like -target and --sysroot to be set for
architecture testing and compilation.

Reviewers: tberghammer, srhines, danalbert, beanz, compnerd

Subscribers: tberghammer, llvm-commits, danalbert

Differential Revision: https://reviews.llvm.org/D22415

llvm-svn: 276333