Ensure -mcpu=xscale works for arm targets, after rL252903 and rL252904

After these revisions, for arm targets, the -mcpu=xscale option caused
an error: "the clang compiler does not support '-mcpu=xscale'".  Adding
"v5e" as a SUB_ARCH in ARMTargetParser.def helps.

Submitted by: Andrew Turner
Differential Revision: http://reviews.llvm.org/D16043

llvm-svn: 257376