linux-user: Check type of microMIPS break instruction

microMIPS instructions that cause breakpoint exceptions come in
16-bit and 32-bit variants.  When handling exceptions caused by
such instructions, the instruction type needs to be taken into
account when extracting the break code.

The code has also been restructured for better clarity.

Signed-off-by: Kwok Cheung Yeung <kcy@codesourcery.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
