gdb(4) fix x86 signal reporting

The existing values correspond to x86 exception vector numbers, but the
trap numbers used in the kernel do not match these 1-to-1. Prefer the
definitions from x86/trap.h, as they are what actually get passed to
kdb_trap(). This is of little consequence, as gdb_cpu_signal() only
reports the trap reason (signal number) to the gdb client.

This is limited to the subset of trap values for which kdb_trap() is
reachable.

Reviewed by: kib
Discussed with: jhb
MFC after: 1 week
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27645