linuxboot_dma: compile for i486

The ROM uses the cmovne instruction, which is new in Pentium Pro and does not
work when running QEMU with "-cpu 486".  Avoid producing that instruction.

Suggested-by: Richard W.M. Jones <rjones@redhat.com>
Suggested-by: Thomas Huth <thuth@redhat.com>
Reported-by: Rob Landley <rob@landley.net>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

pc-bios/optionrom/Makefile

@@ -13,6 +13,7 @@ $(call set-vpath, $(SRC_PATH)/pc-bios/optionrom)
 ifeq ($(lastword $(filter -O%, -O0 $(CFLAGS))),-O0)
 override CFLAGS += -O2
 endif
+override CFLAGS += -march=i486
 
 # Drop -fstack-protector and the like
 QEMU_CFLAGS := $(filter -W%, $(QEMU_CFLAGS)) $(CFLAGS_NOPIE) -ffreestanding