cmd/go: limit default darwin/arm parallelism

iOS devices can only run tests serially.

Change-Id: I3f4e7abddf812a186895d9d5138999c8bded698f
Reviewed-on: https://go-review.googlesource.com/5751Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

src/cmd/go/build.go

@@ -62,7 +62,8 @@ and test commands:
 		print the commands but do not run them.
 	-p n
 		the number of builds that can be run in parallel.
-		The default is the number of CPUs available.
+		The default is the number of CPUs available, except
+		on darwin/arm which defaults to 1.
 	-race
 		enable data race detection.
 		Supported only on linux/amd64, freebsd/amd64, darwin/amd64 and windows/amd64.
@@ -117,6 +118,14 @@ func init() {
 
 	addBuildFlags(cmdBuild)
 	addBuildFlags(cmdInstall)
+
+	if buildContext.GOOS == "darwin" && buildContext.GOARCH == "arm" {
+		// darwin/arm cannot run multiple tests simultaneously.
+		// Parallelism is limited in go_darwin_arm_exec, but
+		// also needs to be limited here so go test std does not
+		// timeout tests that waiting to run.
+		buildP = 1
+	}
 }
 
 // Flags set by multiple commands.