Disallow invalid PT_GNU_STACK
Stack must be at least readable and writable.
PR: 242570
Reviewed by: kib, markj
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35867
(cherry picked from commit fbafa98a94cc7f4f30d71003c230d1b5d4c5960b)