rules.mk: set default host tools GCC optimization level to -O2

This patch can help save over 60% of package build time. -O2
optimization level is the default value before the commit
e8b470139c78 ("tools: add options to optimize host binaries").

Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18812
Signed-off-by: Robert Marko <[email protected]>

rules.mk

@@ -278,7 +278,7 @@ PKG_CONFIG:=$(STAGING_DIR_HOST)/bin/pkg-config
 
 export PKG_CONFIG
 
-HOST_FLAGS_OPT:=$(call qstrip,$(CONFIG_HOST_FLAGS_OPT))
+HOST_FLAGS_OPT:=$(if $(CONFIG_HOST_FLAGS_OPT),$(call qstrip,$(CONFIG_HOST_FLAGS_OPT)),"-O2")
 HOST_FLAGS_STRIP:=$(call qstrip,$(CONFIG_HOST_FLAGS_STRIP))
 HOST_EXTRA_CFLAGS:=$(call qstrip,$(CONFIG_HOST_EXTRA_CFLAGS))
 HOST_EXTRA_CXXFLAGS:=$(call qstrip,$(CONFIG_HOST_EXTRA_CXXFLAGS))