ads2gas: Allow converting code to thumb mode

ads2gas: Allow converting code to thumb mode

Currently this only supports thumb2.

This involves rewriting certain instructions that can't be expressed
in thumb2 into equivalent instruction sequences. The regexps for
rewriting are currently written pretty narrowly, only covering the
exact cases currently encountered in the code base.

No IT instructions are added (since that would require more logic than
plain regexps), so using the thumb mode requires enabling
-mimplicit-it=always/thumb.

Change-Id: I1f676ad1d351381f02bcf00105102aa8dd7ae364