[X86] Don't disable xsave when avx is disabled. Implicitly enable xsave with...

[X86] Don't disable xsave when avx is disabled. Implicitly enable xsave with avx is enabled and xsave wasn't explciitly disabled

CPUs with avx always have xsave, but some CPUs without avx also
have xsave. So we shouldn't disable xsave just because avx is
disabled. This would prevent xsave from being enabled with
-march=native on CPUs with xsave and not avx.

But we also don't want -mavx -mno-avx to leave xsave eanabled.
So only enable xsave if avx is enabled after processing all features.

I thought about just not turning xsave on with avx at all, but
there might be someone out there depending on it.