arm: x264_coeff_last8_arm

checkasm --bench on a coretex-a9:
coeff_last8_c: 173
coeff_last8_armv6: 151

60 instead of 73 cycles in ~130k runs on the same cpu while encoding.
configure: remove an unnecessary option from CFLAGS on OS X

Also this commit supported compiling with Clang 3.4 on OS X.
arm: fix compilation for HIGH_BIT_DEPTH

Caused by conflicting prototypes for neon predict functions added in
babcabe433d883e0e916aec4b6c352849931a2eb to arm/predict.h.
Remove the duplicate prototypes from arm/predict-c.c and change the type
from pixel to uint8_t since there are no plans for high bit depth arm
neon optimizations.