Reliably enable debug exceptions on all CPUs.

[jhb]

Summary

Previously, debug exceptions were only enabled on the boot CPU if
DDB was enabled in the dbg_monitor_init() function. APs also called
this function, but since mp_machdep.c doesn't include opt_ddb.h, the
APs ended up calling an empty stub defined in <machine/debug_monitor.h>
instead of the real function. Also, if DDB was not enabled in the kernel,
the boot CPU would not enable debug exceptions.

Fix this by adding a new dbg_init() function that always clears the OS
lock to enable debug exceptions which the boot CPU and the APs call.
This function also calls dbg_monitor_init() to enable hardware breakpoints
from DDB on all CPUs if DDB is enabled. Eventually base support for
hardware breakpoints/watchpoints will need to move out of the DDB-only
debug_monitor.c for use by userland debuggers.

Test Plan

• cpuset -l 0 gdb /bin/ls and single step would work
• cpuset -l 1 gdb /bin/ls and single step would not

[jhb]

sys/arm64/arm64/machdep.c

225 ↗

(On Diff #31974)

This includes D11972 because I tested these in a shared tree, but I will commit D11972 first.

[andrew]

I think Semihalf was only interested in the kernel debugger when they added this.