bsd.compat.mk Extract _ALL_LIBCOMPATS and add related variables

Adding these to a new bsd.compat.pre.mk will allow other parts of the
tree to iterate over the set of possible libcompats (upper and/or lower)
rather than having to hard-code the list.

Reviewed by: brooks, jhb, emaste
Differential Revision: https://reviews.freebsd.org/D41178