panic stops every other processor

A panic used to stop only the processor it happened on. The rest carried
on with a dead kernel: on the ASUS laptop the report flashed up and was
drawn over, and the fans stayed loud until the power button was held.

Now the panicking processor sends every other one an NMI, which arrives
even with interrupts off, and trap() halts a processor that takes one
while panic_stop is set -- first thing, before the canary check, since
that stack may be what broke. NMI gets its own per-CPU stack (IST 1):
for a few instructions at the top of syscall_entry the live stack is the
user's, at ring 0, and a fault pushing the frame there would be a double
fault on the same stack and a reset instead of a report.

A second processor that panics while one is already reporting halts
instead of drawing over it; the owner's own nested panic still gets
through.

Checked under QEMU with -smp 4: after a sysrq crash CPUs 1-3 sit in hlt
with IF clear and CPU 0 is in the report's blink loop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/core/defs.h

@@ -457,6 +457,8 @@ extern uint64   boot_epoch;
 int             cpunum(void);
 extern volatile uint*    lapic;
 void            lapiceoi(void);
+extern volatile int panic_stop;   // kernel/core/panic.c
+void            lapic_nmi_others(void);
 void            lapicinit(void);
 void            lapicstartap(uchar, uint);
 void            microdelay(int);