cmd/go: add error for cross-compiled -race builds
cmd/go: add error for cross-compiled -race builds

Race builds require C dependencies, but cross-compiled cgo builds are
not always possible, so don't suggest enabling CGO in those cases.

Fixes #37021

Change-Id: I1fd675efc9cef958a926bd63eac8e6858bc59d0a
GitHub-Last-Rev: cbf43c1bbb0f209474cc323b3813cf270a2ba0a8
GitHub-Pull-Request: golang/go#38670
Reviewed-on: https://go-review.googlesource.com/c/go/+/230202
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/go,nawawi/go,golang/go,akutz/go,akutz/go,nawawi/go,nawawi/go,akutz/go,nawawi/go,golang/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go