[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@354622 91177308-0d34-0410-b5e6-96231b3b80d8