ARMv6 optimized subtract functions

Adds following ARMv6 optimized functions to encoder:
  - vp8_subtract_b_armv6
  - vp8_subtract_mby_armv6
  - vp8_subtract_mbuv_armv6

Gives 1-5% speed-up depending on input sequence and encoding
parameters. Functions have one stall cycle inside the loop body
on Cortex pipeline.

Change-Id: I19cca5408b9861b96f378e818eefeb3855238639