Improve lookahead-threads auto selection
Smarter decision to improve fast-first-pass performance in 2-pass encodes.
Dramatically improves CPU utilization on multi-core systems.

Tested on a quad-core Ivy Bridge (12 threads, 1080p):
Fast first pass:
veryfast:     ~7% faster
faster:      ~11% faster
fast/medium: ~15% faster
slow/slower: ~42% faster
veryslow:    ~55% faster
CRF/1-pass:
veryfast:     ~9% faster
(all others remained the same)
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