Unify CPU pragmas in a common header?

[icculus]

Also, It would probably make sense to wrap the pragmas needed with GCC and Clang into a define in the SDL_cpuinfo.h header (?) and deploy them to other places were intrinsics are used (e.g. SDL_ConvertStereoToMono). I refrained from doing so myself because I am certain I would at least break one of the build system/platform combinations :P

Originally posted by @JoelLinn in #4436 (comment)

[sezero]

AVX build failure issue noted here: #4436 (comment)
@JoelLinn mentioned this issue entry

[JoelLinn]

Another thing brought to my attention (thx @Triang3l) is constant initialized vector values. Under some circumstances the compiler might generate init code with instruction set extensions that will run on program start even though the specialized code would never be branched into (because the CPU lacks the features). Since we don't enable the extensions for the complete translation unit (but just single functions), I hope we won't run across this. Still leaving this here as a reminder.