amd64: Drop segment descriptor details from trap messsages

Segment descriptor contents are fixed on amd64 and not very interesting
compared to the other values that are displayed.

While here, include both the thread and process details of the current
thread and drop redundant output of the trap number.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D56989

(cherry picked from commit 00b96a777845d9b558b2303cbef03ba5197b593a)