Strip Android version when looking up toolchain paths.
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