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>
(cherry picked from commit 7e018385)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>