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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.
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