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.
Check in configure whether the compiler supports the flags for disabling
PIE, and if it does then use them for building the ROMs. This fixes the
following buildbot failure:

>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>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>