Complete the existing support of ARM v6m, v7m, and v7em, i.e., respectively cortex-m0, cortex-m3, and cortex-m4 on the backend side.

Adds new subtype values for the MachO format and use them when the related triple are set.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172472 91177308-0d34-0410-b5e6-96231b3b80d8