sound: Improve afmt_tab

Reduce ifdefs, and add aliases for the unsigned formats.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D48009

(cherry picked from commit a4aff024fd53a38ba08bbf5309589e1865ffe024)