gcc: build with HOST_CFLAGS

Turns on -O2 optimization when building GCC thus making the cross-compiler a
little faster (about 25%).

Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 34266

toolchain/gcc/common.mk

@@ -180,6 +180,7 @@ endif
 GCC_MAKE:= \
 	export SHELL="$(BASH)"; \
 	$(MAKE) \
+		CFLAGS="$(HOST_CFLAGS)" \
 		CFLAGS_FOR_TARGET="$(TARGET_CFLAGS)" \
 		CXXFLAGS_FOR_TARGET="$(TARGET_CFLAGS)"