Deduplicate asm constants, automate name prefixing
Auto-prefix global constants with x264_ in cextern. Eliminate x264_ prefix from asm files; automate it in cglobal. Deduplicate asm constants wherever possible to save data cache (move them to a new const-a.asm). Remove x264_emms() entirely on non-x86 (don't even call an empty function). Add cextern_naked for a non-prefixed cextern (used in checkasm).
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