Emit diagnostic for -munaligned-access on v6m targets

Rather than silently disabling unaligned accesses for v6m targets as
in the previous patch to llvm, instead produce a warning saying that
this architecture doesn't support unaligned accesses.

Patch by Ben Foster



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215531 91177308-0d34-0410-b5e6-96231b3b80d8