[Driver] Default all Android ARM targets to NEON.

Summary:
There are an insignificant number of ARM Android devices that don't
support NEON. Default to using NEON since that will improve
performance on the majority of devices. Users that need to target
non-NEON devices can still explicitly disable NEON.

Reviewers: srhines, pirama, kristof.beyls

Reviewed By: pirama

Subscribers: efriedma, javed.absar, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58153

llvm-svn: 354166