Makefile.inc1: synthesize PKG_ABI from newvers.sh variables

Previously we inspected ${WSTAGEDIR}/usr/bin/uname to determine PKG_ABI,
but the file will not exist in some cases - for example, if building
only kernel packages. We can instead synthesize the PKG_ABI from
information already provided by newvers.sh.

Reviewed by: kevans, manu (both earlier rev)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34249