x86: implement kdb watchpoint functions

Add wrappers around the dbreg interface that can be consumed by MI
kernel debugger code. The dbreg functions themselves are updated to
return error codes, not just -1. dbreg_set_watchpoint() is extended to
accept access bits as an argument.

Reviewed by: jhb, kib, markj
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.

(cherry picked from commit 15dc1d44528a1e2693df41d5452d6ebb42ecafeb)