cdboot: add explict suffix to ambiguous or instruction

Clang disallows ambiguous instructions (GNU as has a default based on
chosen .code setting). We only need 'orb' here because KARGS_FLAGS_PXE
fits in a byte; this is the same as done in bxeboot.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8959