[ARM] Alter test to account for change to armv6k default CPU

Review D52594 will change the default in llvm for armv6k from the
non-existent cpu arm1176jf-s to mpcore. The tests in arm-cortex-cpus.c
need to be updated to account for this change.

Differential Revision: https://reviews.llvm.org/D52595

llvm-svn: 343304