OpenCL: Reduce binary size

The source code of the OpenCL kernels is stored as a string in the x264 binary
and compiled at run time. Shrink this string by removing as many redundant
non-token characters as possible.

Reduces the x264 binary size by 20 kB.
checkasm: Only call x264_cpu_detect() once
x86: Faster quant_4x4x4

Also drop the MMX version instead of doing a bunch of ifdeffery to support it after this change.
x86: Minor pixel_ssim_end4 improvements

Reduce the number of vector registers used from 7 to 5.
Eliminate some moves in the AVX implementation.
Avoid bypass delays for transitioning between int and float domains.
x86inc: Make INIT_CPUFLAGS support an arbitrary number of cpuflags

Previously there was a limit of two cpuflags.