Split CommonGlobalIncludes for system includes

Split CommonGlobalSystemIncludes out of CommonGlobalIncludes in
preparation for moving global includes from -isystem to -I.

Bug: 31492149
Change-Id: Ib935ea038cdbf9515dc2ab68d7fff924c370906a
diff --git a/cc/compiler.go b/cc/compiler.go
index 4a7bba9..37dc744 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -147,6 +147,7 @@
 		if !ctx.sdk() || ctx.Host() {
 			flags.GlobalFlags = append(flags.GlobalFlags,
 				"${config.CommonGlobalIncludes}",
+				"${config.CommonGlobalSystemIncludes}",
 				tc.IncludeFlags(),
 				"${config.CommonNativehelperInclude}")
 		}