Switch CPU names not recognized by GNU assembler

Summary:
Switch CPU names not recognized by GNU assembler to a close CPU that it
does recognize.  In this patch, kryo, falkor and saphira all get
replaced by cortex-a57 when invoking the assembler.  In addition, krait
was already being replaced by cortex-a15.

Reviewers: weimingz

Subscribers: srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D40476

llvm-svn: 319077