openbsd: adding EPERM for msync() against hitting a immutable region page

[rwsalie]

OpenBSD, currently, will not let the permission to msync() (which is mapping destructive) hit another page marked as immutable. If msync() hit a region immutable he will return EPERM.

more information at: https://marc.info/?l=openbsd-cvs&m=170585617027230&w=2

I've been triggered by this error during stage3 which cause the unreachable (below) #18696

This commit, will let openbsd oses compiling well :smile:

[semarie]

from OpenBSD point of vue, it LGTM.