soc/cavium: Guard gcc specific compiler flag

TEST: BUILD_TIMELESS=1 remains the same.

Change-Id: I740b59574303145fc673a97556367daefe8d1540
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74540
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/cavium/common/Makefile.inc

@@ -5,7 +5,9 @@ ifeq ($(CONFIG_SOC_CAVIUM_COMMON),y)
 subdirs-y += pci
 
 # -Wstack-usage doesn't recognize hexadecimal numbers.
+ifeq ($(CONFIG_COMPILER_GCC),y)
 CFLAGS_arm64   += -Wstack-usage=$(shell printf "%d" $(CONFIG_STACK_SIZE))
+endif
 
 bootblock-$(CONFIG_BOOTBLOCK_CUSTOM) += bootblock.c