Switch krait to use -mcpu=cortex-a15 for assembler tool invocations.

Summary:
Using -no-integrated-as causes -mcpu=krait to be transformed into
-march=armv7-a today. This precludes the assembler from using
instructions like sdiv, which are present for krait. Cortex-a15 is the
closest subset of functionality for krait, so we should switch the
assembler to use that instead.

Reviewers: cfe-commits, apazos, weimingz

Subscribers: aemerson

Differential Revision: http://reviews.llvm.org/D17874

llvm-svn: 262742