Add x86-64-v2, x86-64-v3, and x86-64-v4 as available target_cpus

[hsivonen]

Page 14 of the 2020-08-31 version of the AMD64 supplement to the System V ABI defines new microarchitecture levels that bundle instruction set extensions: x86-64-v2, x86-64-v3, and x86-64-v4. According to a Red Hat blog post, LLVM 12 will support these.

rustc should support these as target_cpu values, enable the right target_features, and pass them appropriately to LLVM.

Internals thread

[IsaacWoods]

The link to the internals thread also points to RedHat's blog. I'm guessing this is the internals thread.

[hsivonen]

Oops. Updated the link. Thanks.

[ojeda]

rustc does not manage these options, LLVM does. Thus when we move to LLVM 12, we will get these.

[est31]

LLVM commit for reference: llvm/llvm-project@012dd42e027e

[est31]

Also I confirm what @ojeda said. In #79043, all I had to do was patch LLVM to add availability of these target_cpus. No changes in rustc were neccessary.

[jyn514]

The compiler is now using LLVM 12.0.1, so this might be fixed?

[hsivonen]

Indeed, this is now fixed. Thanks.