ARM: fix regression caused by r198914

The disassembler would no longer be able to disambiguage between the two
variants (explicit immediate #0 vs implicit, omitted #0) for the ldrt, strt,
ldrbt, strbt mnemonics as both versions indicated the disassembler routine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198944 91177308-0d34-0410-b5e6-96231b3b80d8
Must not produce Tag_CPU_arch_profile for pre-ARMv7 cores (e.g. cortex-m0)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198945 91177308-0d34-0410-b5e6-96231b3b80d8
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