Lyndon Brown authored Jan 26, 2019 and François Cartegnie committed Jul 20, 2021

notes:
 - this removes all SIMD acceleration for x86/x86_64. originally this work
   started by converting the MMX code to SSE2, then purged remaining
   artifacts, but a build error on android has blocked that work from
   being merged for now. this commit thus takes a different approach of
   simply purging the old MMX/MMXEXT code first, with getting the SSE2
   implementation working to be done as a follow up.
 - the `EndMMX()` function is retained (renamed to `EndSSE()`) because it
   is still used under the merge code. the `emms` instruction will be
   replaced with an `sfence` instruction separately, as more appropriate.