Update X86 feature and CPU detection

[yuyichao]

First three commits assumes LLVM >= 5.0
Last two commits assumes LLVM >= 7.0 (really 8.0 but nothing was needed from 8.0)

At least the first commit should be backported to all versions that supports LLVM 8.0 ref #35215 (comment)

[KristofferC]

Based on

First three commits assumes LLVM >= 5.0
Last two commits assumes LLVM >= 7.0 (really 8.0 but nothing was needed from 8.0)

I assume all 5 commits should be backported to 1.5 (so I did that in #36577). You could take a look if you want to see that it looks reasonable, there was only one fairly trivial (hopefully) merge conflict.

[yuyichao]

All 5 should be safe, just that the latter 3 has a higher chance of mistake/introducing errors....

6fd5b82 seems to be missing? That's actually the only bugfix that should be backported even if others don't. That's probablly also the origin of the conflict.

[KristofferC]

6fd5b82 seems to be missing?

Hm, yes. So I did mess something up. Will fix it.

[KristofferC]

Added

[KristofferC]

I thought about it a bit and maybe it is better to just backport the bugfix commit since we are so late in the release cycle. I changed the backport branch to that.