go: support configuring "micro-architectures" for instruction set selection

[tdyas]

Go supports configuring a "micro-architecture" via environment variables specific to each GOARCH. This allows selecting variants/extenions of the architecture's instruction set, for example. (And there may be other uses.) See golang/go#45453 for x86-64 example.

Pants should pass through those environment variables or support setting them via a config option. They will also need to be incorporated into the cache key.

[tdyas]

https://github.com/golang/go/blob/21998413ad82655fef1f31316db31e23e0684b21/src/cmd/go/internal/cfg/cfg.go#L261-L312 is the relevant cache key code.