ddb: de-duplicate decode_syscall()

Only i386 and amd64 print the decoded syscall name in the backtrace.
This de-duplication facilitates further changes and adoption by other
platforms.

Reviewed by: jrtc27, markj, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36565

(cherry picked from commit 754cb545b68ba0a1643792763d000018ffe2afec)
(cherry picked from commit 6f8a182b15c4c14cfb9462b806a928ddaf47c87e)
(cherry picked from commit 89e5ef8917af900558c2d275f5d5c4c703520ead)
(cherry picked from commit 85b715baae22ca12f2d24e1b2980f3d5b32937b8)