When a Cortex-M3 target is selected (armv7m), force disassembly to be in
thumb mode.
<rdar://problem/14107444>, <rdar://problem/14107405>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@186881 91177308-0d34-0410-b5e6-96231b3b80d8