The optionrom build is disregarding the flags passed to the configure
script via --cross-cflags-i386. Pass it down and add it to the Makefile.
This will make it possible to get the -m32 flag from $target_cflags to
force a 32-bit build on 64-bit hosts, instead of supplying manually the
arcane -Wa,-32 and linker emulation options.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>