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

[dim]

Summary

To make it possible to use .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.

NOTE: split off from D49962.

MFC after: 3 days

[emaste]

I got my bikeshedding out of the way on other reviews so will reserve comment on the filename :)

[dim]

In D49965#1139483, @emaste wrote:

I got my bikeshedding out of the way on other reviews so will reserve comment on the filename :)

Yeah, it can't be bsd.suffixes.mk since that's already taken, and there is already a bsd.suffixes-posix.mk. I didn't have too much inspiration so bsd.suffixes-extra.mk seemed reasonable enough. I don't think there will be much need for _more_ of these bsd.suffixes files, at least I hope so!