cmd/go: import runtime/cgo into darwin/arm64 tests

cmd/go: import runtime/cgo into darwin/arm64 tests

Until cl/12721 and cl/12574, all standard library tests included
runtime/cgo on darwin/arm64 by virtue of package os including it. Now
that is no longer true, runtime/cgo needs to be added by the go tool
just as it is for darwin/arm. (This installs the Mach exception
handler used to properly handle EXC_BAD_ACCESS.)

Fixes #11901

Change-Id: I991525f46eca5b0750b93595579ebc0ff10e47eb
Reviewed-on: https://go-review.googlesource.com/12723
Reviewed-by: Russ Cox <rsc@golang.org>