runtime/cgo: no-op getwd call as test breakpoint

This is to be used by an lldb script inside go_darwin_arm_exec to pause
the execution of tests on iOS so the working directory can be adjusted
into something resembling a GOROOT.

Change-Id: I69ea2d4d871800ae56634b23ffa48583559ddbc6
Reviewed-on: https://go-review.googlesource.com/6363Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/cgo/gcc_darwin_arm.c

@@ -123,6 +123,11 @@ init_working_dir()
 	if (chdir(dir) != 0) {
 		fprintf(stderr, "runtime/cgo: chdir(%s) failed\n", dir);
 	}
+
+	// No-op to set a breakpoint on, immediately after the real chdir.
+	// Gives the test harness in go_darwin_arm_exec (which uses lldb) a
+	// chance to move the working directory.
+	getwd(dir);
 }
 
 void