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).