fix remaining CFLAGS_GCC references in example Makefiles

CFLAGS_GCC is no longer consumed after IAR make removal. Change to
CFLAGS in net_lwip_webserver, host_hid_to_device_cdc, and
host_info_to_device_cdc so warning suppression flags are applied.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hathach

examples/device/net_lwip_webserver/Makefile

@@ -1,7 +1,7 @@
 include ../../../hw/bsp/family_support.mk
 
 # suppress warning caused by lwip
-CFLAGS_GCC += \
+CFLAGS += \
   -Wno-error=null-dereference \
   -Wno-error=unused-parameter \
   -Wno-error=unused-variable

examples/dual/host_hid_to_device_cdc/Makefile

@@ -8,7 +8,7 @@ INC += \
 EXAMPLE_SOURCE += $(wildcard src/*.c)
 SRC_C += $(addprefix $(EXAMPLE_PATH)/, $(EXAMPLE_SOURCE))
 
-CFLAGS_GCC += -Wno-error=cast-align -Wno-error=null-dereference
+CFLAGS += -Wno-error=cast-align -Wno-error=null-dereference
 
 SRC_C += \
 	src/class/hid/hid_host.c \

examples/dual/host_info_to_device_cdc/Makefile

@@ -8,7 +8,7 @@ INC += \
 EXAMPLE_SOURCE += $(wildcard src/*.c)
 SRC_C += $(addprefix $(EXAMPLE_PATH)/, $(EXAMPLE_SOURCE))
 
-CFLAGS_GCC += -Wno-error=cast-align -Wno-error=null-dereference
+CFLAGS += -Wno-error=cast-align -Wno-error=null-dereference
 
 SRC_C += \
 	src/host/hub.c \