[Thumb2] ldrexd and strexd are not defined on v7M

The Thumb2 ldrexd and strexd instructions are not defined for
M-class architectures.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218603 91177308-0d34-0410-b5e6-96231b3b80d8