Fix the missing 2nd arch prefix.

Bug: 13751317
Change-Id: Ie5a323a0a89245576dbc91271d3178574942627d
diff --git a/core/combo/HOST_darwin-x86.mk b/core/combo/HOST_darwin-x86.mk
index d665c76..4a2bfe3 100644
--- a/core/combo/HOST_darwin-x86.mk
+++ b/core/combo/HOST_darwin-x86.mk
@@ -49,7 +49,7 @@
 
 # gcc location for clang; to be updated when clang is updated
 # HOST_TOOLCHAIN_ROOT is a Darwin-specific define
-HOST_TOOLCHAIN_FOR_CLANG := $(HOST_TOOLCHAIN_ROOT)
+$(combo_2nd_arch_prefix)HOST_TOOLCHAIN_FOR_CLANG := $($(combo_2nd_arch_prefix)HOST_TOOLCHAIN_ROOT)
 
 $(combo_2nd_arch_prefix)HOST_AR := $(AR)