cmd/compile: unsafe.{Add,Slice} should require -lang=go1.17

[mdempsky]

Go 1.17 added unsafe.Add and unsafe.Slice. They shouldn't be available to packages targeting older releases.

[mdempsky]

/cc @ianlancetaylor to confirm this.

We don't otherwise enforce that -lang prevents users from using new functionality available in imports, but package unsafe is specially enshrined in the Go spec, so it seems like a case we should detect and emit an error for.

[gopherbot]

Change https://golang.org/cl/324369 mentions this issue: cmd/compile: restrict use of unsafe.{Add,Slice} to go1.17 or newer

[ianlancetaylor]

Seems reasonable to me.