cmd/go: declare runtime/cgo dependency for darwin/arm, darwin/arm64 tests

cmd/go: declare runtime/cgo dependency for darwin/arm, darwin/arm64 tests

I don't know why these tests must import runtime/cgo
in _testmain.go, but if they must, they must also tell the
rest of the go command that they are doing so.

Should fix the newly-broken darwin/arm and darwin/arm64 builders.

Change-Id: I9b183f8c84c6f403bf3a90cbfc838d6ef428e16f
Reviewed-on: https://go-review.googlesource.com/67230
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>