cmd/dist: ensure consistency between (*tester).supportedBuildmode and cmd/internal/sys.BuildModeSupported

[bcmills]

(*cmd/dist.tester).supportedBuildmode contains a large switch that determines whether a given buildmode is supported for a given GOOS/GOARCH:

go/src/cmd/dist/test.go

Lines 977 to 1038 in fa90aac

	func (t *tester) supportedBuildmode(mode string) bool {
	pair := goos + "-" + goarch
	switch mode {
	case "c-archive":
	if !t.extLink() {
	return false
	}
	switch pair {
	case "aix-ppc64",
	"darwin-amd64", "darwin-arm64", "ios-arm64",
	"linux-amd64", "linux-386", "linux-ppc64le", "linux-s390x",
	"freebsd-amd64",
	"windows-amd64", "windows-386":
	return true
	}
	return false
	case "c-shared":
	switch pair {
	case "linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le", "linux-s390x",
	"darwin-amd64", "darwin-arm64",
	"freebsd-amd64",
	"android-arm", "android-arm64", "android-386",
	"windows-amd64", "windows-386":
	return true
	}
	return false
	case "shared":
	switch pair {
	case "linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le", "linux-s390x":
	return true
	}
	return false
	case "plugin":
	// linux-arm64 is missing because it causes the external linker
	// to crash, see https://golang.org/issue/17138
	switch pair {
	case "linux-386", "linux-amd64", "linux-arm", "linux-s390x", "linux-ppc64le":
	return true
	case "darwin-amd64", "darwin-arm64":
	return true
	case "freebsd-amd64":
	return true
	}
	return false
	case "pie":
	switch pair {
	case "aix/ppc64",
	"linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le", "linux-riscv64", "linux-s390x",
	"android-amd64", "android-arm", "android-arm64", "android-386":
	return true
	case "darwin-amd64", "darwin-arm64":
	return true
	case "windows-amd64", "windows-386", "windows-arm":
	return true
	}
	return false
	default:
	fatalf("internal error: unknown buildmode %s", mode)
	return false
	}
	}

That switch nearly exactly duplicates the one in cmd/internal/sys.BuildModeSupported, which was itself extracted from a portion of cmd/go/internal/work.buildModeInit.

cmd/dist should be changed to use (instead of duplicating) the function from cmd/internal/sys.

[ianlancetaylor]

My understanding is that cmd/dist needs to build with Go 1.4, so it can't import cmd/internal/sys. That's why I wrote two versions initially.

[bcmills]

Ah, that could be.

But in that case, we should still have something in place that ensures that the two switches are equivalent, such as a regression-test that compares the two functions.

Or, perhaps (*tester).supportedBuildmode should probe cmd/go to determine support instead of encoding its own parallel assumptions in-process.