This patch adds basic support for the Thumb ADDW and SUBW instructions.

The patch permits the compiler to use the new instructions for constants 
that can be loaded with a single instruction (i.e. 16-bit unshifted), 
but does not support use of addw with split-constants; I have a patch 
for that coming soon.

This patch requires that my previously posted patch for MOVW is applied 
first.

OK?

Andrew