libpayload: Skip unknown arguments to clang

This compiler argument only exists on gcc.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Change-Id: I10902517c86daedc9853e6f6cac8fcf513211bb2
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77436
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/arch/x86/Makefile.inc

@@ -26,7 +26,9 @@
 ## SUCH DAMAGE.
 ##
 
+ifneq ($(CONFIG_LP_COMPILER_LLVM_CLANG),y)
 CFLAGS += -mpreferred-stack-boundary=2
+endif
 
 head.o-y += head.S
 libc-y += main.c sysinfo.c