Add support for macOS 11 SDK

Test: try building on the xcode 12 beta 2 macOS 11 SDK
Change-Id: Ic7e245ca418914798ba737106c6a345844e7a6ac
diff --git a/cc/config/x86_darwin_host.go b/cc/config/x86_darwin_host.go
index 8eb79e3..81c907d 100644
--- a/cc/config/x86_darwin_host.go
+++ b/cc/config/x86_darwin_host.go
@@ -66,6 +66,7 @@
 		"10.13",
 		"10.14",
 		"10.15",
+		"11.0",
 	}
 
 	darwinAvailableLibraries = append(