Add -mpopcnt for X86_64 targets if they support popcnt

Change-Id: I09668a220f207d3d7510f058610846e4427d815e
diff --git a/cc/x86_64_device.go b/cc/x86_64_device.go
index 24c444e..ceaf51c 100644
--- a/cc/x86_64_device.go
+++ b/cc/x86_64_device.go
@@ -88,6 +88,7 @@
 		"sse4":   []string{"-msse4"},
 		"sse4_1": []string{"-msse4.1"},
 		"sse4_2": []string{"-msse4.2"},
+		"popcnt": []string{"-mpopcnt"},
 		"avx":    []string{"-mavx"},
 		"aes_ni": []string{"-maes"},
 	}