Fix QEMU build on OpenBSD on x86 archs

This resolves the build issue with building the ROMs on OpenBSD on x86 archs.
As of OpenBSD 5.3 the compiler builds PIE binaries by default and thus the
whole OS/packages and so forth. The ROMs need to have PIE disabled. This
is my initial attempt at trying to get somehting upstream so that QEMU
both builds out of the box and to resolve the build issue with the
buildbots that has been around for awhile. We have a patch in our ports
tree but it is just the flags hardcoded into the Makefile which obviously
is not appropriate for upstream.

From the OpenBSD buildbots..
  Building optionrom/multiboot.img
ld: multiboot.o: relocation R_X86_64_16 can not be used when making a shared object; recompile with -fPIC

Signed-off by: Brad Smith <brad@comstyle.com>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>