Bug 1585351 - Bug 1460357 disabled AVX instructions for the mingw build; this...

Bug 1585351 - Bug 1460357 disabled AVX instructions for the mingw build; this is no longer needed r=lsalzman

Bug 1460357 disabled AVX because gcc was generating unaligned instructions. But clang doesn't seem
to do that.

Differential Revision: https://phabricator.services.mozilla.com/D48072

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extra : moz-landing-system : lando