cmd/link: support c-archive for freebsd/arm and freebsd/arm64

[hajimehoshi]

This is a request. The buildmode c-archive is not supported on freebsd/arm and freebsd/arm64 yet (go1.16-beta)

go/src/cmd/link/internal/ld/config.go

Lines 57 to 75 in cb84d83

	case "c-archive":
	switch objabi.GOOS {
	case "aix", "darwin", "ios", "linux":
	case "freebsd":
	switch objabi.GOARCH {
	case "amd64":
	default:
	return badmode()
	}
	case "windows":
	switch objabi.GOARCH {
	case "amd64", "386", "arm":
	default:
	return badmode()
	}
	default:
	return badmode()
	}
	*mode = BuildModeCArchive