Move extra suffix-transformation rules into bsd.suffixes-extra.mk

To make it possible to use the .SUFFIXES list and suffix-transformation
rules from bsd.lib.mk in other places, move them to a separate file,
bsd.suffixes-extra.mk.

Note that we cannot add the list and rules to bsd.suffixes.mk, since
that file also gets included by sys.mk, which applies to non-source
builds. That would require a whole ports exp-run.

No functional change intended.

Reviewed by: brooks, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D49965