Support macOS 12 SDK

Test: build w/Xcode 13.1
Change-Id: I9062b8045c4b02551aeeff06f4d7416c766e53a6
diff --git a/cc/config/darwin_host.go b/cc/config/darwin_host.go
index 526ee52..206bec1 100644
--- a/cc/config/darwin_host.go
+++ b/cc/config/darwin_host.go
@@ -54,6 +54,7 @@
 
 	darwinSupportedSdkVersions = []string{
 		"11",
+		"12",
 	}
 
 	darwinAvailableLibraries = append(