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 <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

christopher-henderson/Go,akutz/go,codestation/go,nawawi/go,Cofyc/go,cloudflare/go,golang/go,Cofyc/go,Cofyc/go,CAFxX/go,polyverse-security/go,codestation/go,josharian/go,polyverse-security/go,Cofyc/go,josharian/go,CAFxX/go,codestation/go,josharian/go,josharian/go,akutz/go,akutz/go,cloudflare/go,Cofyc/go,cloudflare/go,codestation/go,cloudflare/go,Cofyc/go,golang/go,cloudflare/go,nawawi/go,akutz/go,polyverse-security/go,codestation/go,golang/go,CAFxX/go,codestation/go,Cofyc/go,akutz/go,golang/go,golang/go,nawawi/go,josharian/go,codestation/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,polyverse-security/go,golang/go,nawawi/go,polyverse-security/go,codestation/go,CAFxX/go,polyverse-security/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go,Cofyc/go,josharian/go,CAFxX/go,cloudflare/go,josharian/go,josharian/go,nawawi/go,polyverse-security/go,nawawi/go,nawawi/go,nawawi/go,polyverse-security/go,golang/go,golang/go