[Driver] Fix float ABI default for Android ARMv8.

Summary: Android doesn't regress back to soft float after ARMv7 :)

Reviewers: srhines, pirama

Reviewed By: srhines, pirama

Subscribers: javed.absar, kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58477

llvm-svn: 354622