Turns out 64 is not enough to completely prevent hangs on Ampere Altra
with 80 CPUs.

Instead of using a magic number pick the number of CPUs online.  On the
Altra with 80 CPUs this makes the upper bound at 128.  Using the number
of available CPUs should also reduce latency on smaller SMP machines.

With this I can purposely generate contention on the Ampere machine.

ok?

Index: kern/kern_lock.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_lock.c,v
diff -u -p -r1.78 kern_lock.c
--- kern/kern_lock.c	31 May 2025 10:24:50 -0000	1.78
+++ kern/kern_lock.c	3 Jun 2025 08:25:31 -0000
@@ -37,6 +37,8 @@
 int __mp_lock_spinout = INT_MAX;
 #endif /* MP_LOCKDEBUG */
 
+extern int ncpusfound;
+
 /*
  * Min & max numbers of "busy cycles" to waste before trying again to
  * acquire a contended lock using an atomic operation.
@@ -50,7 +52,7 @@ int __mp_lock_spinout = INT_MAX;
  * enough to reduce (ideally avoid) cache line contention.
  */
 #define CPU_MIN_BUSY_CYCLES	1
-#define CPU_MAX_BUSY_CYCLES	64
+#define CPU_MAX_BUSY_CYCLES	ncpusfound
 
 #ifdef MULTIPROCESSOR