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.