Deduplicate loader vers.c Makefile rules

The Makefile rule to create vers.c for loader version info was
previously duplicated in each of the various loader Makefiles.
Instead, share a common rule in Makefile.inc.

Reviewed by: bapt
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8841