amd64: Avoid enabling interrupts when handling kernel mode prot faults

When PTI is enabled, we may have been on the trampoline stack when iret
faults. So, we have to switch back to the regular stack before
re-entering trap().

trap() has the somewhat strange behaviour of re-enabling interrupts when
handling certain kernel-mode execeptions. In particular, it was doing
this for exceptions raised during execution of iret. When switching
away from the trampoline stack, however, the thread must not be migrated
to a different CPU. Fix the problem by simply leaving interrupts
disabled during the window.

Reported by: syzbot+6cfa544fd86ad4647ffc@syzkaller.appspotmail.com
Reported by: syzbot+cfdfc9e5a8f28f11a7f5@syzkaller.appspotmail.com
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30578