h264: new assembly version of get_cabac for x86_64 with PIC

This adds a hand-optimized assembly version for get_cabac much like the
existing one, but it works if the table offsets are RIP-relative.
Compared to the non-RIP-relative version this adds 2 lea instructions
and it needs one extra register. get_cabac() gets about 40% faster, for
an overall speedup of about 5%.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>