Stop test/CodeGen/ARM/a15.ll targetting non-ARM targets.

Summary:
Fixes an issue where a test attempts to use -mcpu=cortex-a15 on non-ARM targets.
This triggers an assertion on MIPS since it doesn't know what ABI to use by default for
unrecognized processors.

Reviewers: rengolin

Reviewed By: rengolin

CC: llvm-commits, aemerson, rengolin

Differential Revision: http://llvm-reviews.chandlerc.com/D2876