[Driver] Fix --hash-style choice for Android.

Summary:
Android supports GNU style hashes as of Marshmallow, so we should be
generating both styles for pre-M targets and GNU hashes for newer
targets.

Reviewers: srhines, pirama

Reviewed By: srhines

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53118

llvm-svn: 344293