Add support for ARMv7-M architecture which uses the Thumb 2 ISA (unified syntax)

Patch by Jérémie Faucher-Goulet!

Differential Revision: https://reviews.llvm.org/D60417

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@358642 91177308-0d34-0410-b5e6-96231b3b80d8