Introduce kdb-level watchpoint functions

This basically mirrors what already exists in ddb, but provides a
slightly improved interface. It allows the caller to specify the
watchpoint access type, and returns more specific error codes to
differentiate failure cases.

This will be used to support hardware watchpoints in gdb(4).

Stubs are provided for architectures lacking hardware watchpoint logic
(mips, powerpc, riscv), while other architectures are added individually
in follow-up commits.

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

(cherry picked from commit 763107f26c3c3f4c8bb314a7cabc0a5548abff03)