Amending test/MC/ARM/thumb2-mclass.s to match its apparent original purpose (to test the ARMv6M/ARMv7M commonality), and creating a new test case for the differences between ARMv6M and ARMv7M

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198946 91177308-0d34-0410-b5e6-96231b3b80d8