build: disable fcf-protection on -march=486 -m16

Some of the roms build with -march=i486 -m16 which is incompatible
with -fcf-protection. That in turn is can be set by default, for
example in Ubuntu [1].
That causes:
 cc1: error: ‘-fcf-protection’ is not compatible with this target

This won't work on -march=i486 -m16 and no matter if set or not we can
override it to "none" if the option is known to the compiler to be
able to build reliably.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/889

[1]: https://wiki.ubuntu.com/ToolChain/CompilerFlags#A-fcf-protection

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220323090713.1002588-1-christian.ehrhardt@canonical.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

pc-bios/optionrom/Makefile

@@ -14,6 +14,10 @@ cc-option = $(if $(shell $(CC) $1 -c -o /dev/null -xc /dev/null >/dev/null 2>&1
 
 override CFLAGS += -march=i486 -Wall
 
+# If -fcf-protection is enabled in flags or compiler defaults that will
+# conflict with -march=i486
+override CFLAGS += $(call cc-option, -fcf-protection=none)
+
 # Flags for dependency generation
 override CPPFLAGS += -MMD -MP -MT $@ -MF $(@D)/$(*F).d