amd64: handle #AC in kernel mode

Recover from it if PCB_ONFAULT handler is provided.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59815

sys/amd64/amd64/trap.c

@@ -532,6 +532,14 @@ trap(struct trapframe *frame)
 			return;
 
 		switch (type) {
+		case T_ALIGNFLT:
+			if (curpcb->pcb_onfault != NULL) {
+				frame->tf_rip = (long)curpcb->pcb_onfault;
+				return;
+			}
+			trap_fatal(frame, 0);
+			return;
+
 		case T_PAGEFLT:			/* page fault */
 			(void)trap_pfault(frame, false, NULL, NULL);
 			return;