Address calculations moved from encodemb_arm.c file to neon
optimized assembly function to save cycles in function calls.
- vp8_subtract_b_neon_func replaced with vp8_subtract_b_neon
that contains all needed address calculations
- unnecessary file encodemb_arm.c removed
- consistent with ARMv6 optimized version
Change-Id: I6cbc1a2670b56c2077f59995fcf8f70786b4990b