Commit v7.0.0-1454-g33ab478742 "configure: enable cross-compilation of optionrom" effectively broke building of optionroms on anything but real i386 or using newly supported cross/docker image. Before that, it was trivial to do that using native x86_64 tools.
# Mac OS X ships with a broken assembler if have_target i386-softmmu x86_64-softmmu && \ test "$targetos" != "darwin" && test "$targetos" != "sunos" && \ test "$targetos" != "haiku" && \ probe_target_compiler i386-softmmu; then subdirs="$subdirs pc-bios/optionrom" ... There, when run on x86_64 system, `probe_target_compiler i386-softmmu' return false immediately, so whole thing gets skipped. FWIW. /mjt