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/makevars.go b/cc/makevars.go
index b07297d..ea32121 100644
--- a/cc/makevars.go
+++ b/cc/makevars.go
@@ -44,7 +44,7 @@
 	ctx.Strict("GLOBAL_CLANG_CPPFLAGS_NO_OVERRIDE", "")
 	ctx.Strict("NDK_PREBUILT_SHARED_LIBRARIES", strings.Join(ndkPrebuiltSharedLibs, " "))
 
-	includeFlags, err := ctx.Eval("${config.CommonGlobalIncludes}")
+	includeFlags, err := ctx.Eval("${config.CommonGlobalIncludes} ${config.CommonGlobalSystemIncludes}")
 	if err != nil {
 		panic(err)
 	}