proposal: internal/cpu: add sha-ni detection for AMD64

[tpaint]

detect sha-ni instructions for AMD64 to support e.g., optimized sha-256 as described in proposal #50543 (#50543)

[randall77]

I don't think we need the proposal process to change internal/cpu. It is internal after all. Just go ahead and change it.

Generally we only add things to internal/cpu when there is a user for them somewhere in the stdlib. Any internal/cpu changes can be part of whatever CL adds that use, or as a separate but associated CL.

[gopherbot]

Change https://go.dev/cl/408794 mentions this issue: internal/cpu: detect sha-ni instruction support for AMD64