pc-bios/optionrom: Fix OpenBSD build with better detection of linker emulation

The various host OSes are irritatingly variable about the name
of the linker emulation we need to pass to ld's -m option to
build the i386 option ROMs. Instead of doing this via a
CONFIG ifdef, check in configure whether any of the emulation
names we know about will work and pass the right answer through
to the makefile. If we can't find one, we fall back to not trying
to build the option ROMs, in the same way we would for a non-x86
host platform.

This is in particular necessary to unbreak the build on OpenBSD,
since it wants a different answer to FreeBSD and we don't have
an existing CONFIG_ variable that distinguishes the two.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sean Bruno <sbruno@freebsd.org>
Message-id: 1470672688-6754-1-git-send-email-peter.maydell@linaro.org
