BROP mitigation on systems without xonly (execute-only) hw-enforcement (OpenBSD)

Additional context, and status about recent developments in OpenBSD.

BROP: https://www.scs.stanford.edu/brop/ (paper "Hacking Blind" (2014): https://www.scs.stanford.edu/brop/bittau-brop.pdf

xonly mitigation status: https://marc.info/?l=openbsd-tech&m=167501519712725&w=2