Strip Android version when looking up toolchain paths.

Summary:
Android target triples can include a version number in the abi field
(e.g. 'aarch64-linux-android21'), used for checking for availability.
However, the driver was searching for toolchain binaries using the
passed in triple as a prefix.

Reviewers: srhines, danalbert, t.p.northover

Subscribers: t.p.northover, aemerson, tberghammer, danalbert, srhines, cfe-commits

Differential Revision: http://reviews.llvm.org/D21163

llvm-svn: 272413