64-bit cabac optimizations

WIN64:
~3% faster and 16 byte shorter cabac_encode_bypass
~8% faster cabac_encode_terminal
~4% faster PIC
Benchmarked on Ivy Bridge

UNIX64:
One instruction less in cabac_encode_bypass
x264.h: improve x264_encoder_reconfig documentation
x86inc: rename program_name to private_prefix
Synced from libav.
The new name is more descriptive and will allow defining a separate public
prefix for externally visible library symbols.
x86inc: add cvisible macro for C functions
Synced from libav.
This allows defining externally visible library symbols.
x86inc: Set ELF hidden visibility for global constants