Adds UXTB16 support to Pixelflinger

 * Add support for UXTB16 to the disassembler
 * Add encoding of the UXTB16 instruction to the Pixelflinger JIT.

Introducing the UXTB16 instruction allows removal of some masking code, and is
beneficial from a pipeline point of view - lots of UXTB16 followed by MUL
sequences.

Also, further rescheduling and use of SMULWB brings extra performance
improvements.

 * Use UXTB16 in bilinear filtered texturing

Uses UXTB16 to extract channels for SIMD operations, rather than creating and
ANDing with masks. Saves a register and is faster on A8, as UXTB16 result can
feed into first stage of multiply, unlike AND.

Also, used SMULWB rather than SMULBB, which allows removal of MOVs used to
rescale results.

Code has been scheduled for A8 pipeline, specifically aiming to allow
multiplies to issue in pipeline 0, for efficient dual issue operation.

Testing on SpriteMethodTest (http://code.google.com/p/apps-for-android/) gives
8% improvement (12.7 vs. 13.7 fps.)

SMULBB to SMULWB trick could be used in <v6 code path, but this hasn't been
implemented.