[Builtins] ARM: Fix msr assembly instruction use for Thumb2.

Summary:
MSR instruction in Thumb2 does not support immediate operand.
Fix this by moving the condition for V7-M to Thumb2 since V7-M support
Thumb2 only. With this change, aeabi_cfcmp.s and aeabi_cdcmp.S files can
be assembled in Thumb2 mode. (This is split out from the review D38227).

Reviewers: compnerd, peter.smith, srhines, weimingz, rengolin, kristof.beyls

Reviewed By: compnerd

Subscribers: aemerson, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D38268

llvm-svn: 314284