Update dirdeps.mk and gendirdeps.mk

The env space consumed by exporting all libc's .meta files
left little room for command line,
so unexport when done.

Update dirdeps.mk to latest and add
dirdeps-targets.mk to simplify/update targets/Makefile

Makefile changes to go with Makefile.depend changes in D22494

Reviewed by: bdrewery
MFC after: 1 week
Sponsored by: Juniper Networks
Differential Revision: https://reviews.freebsd.org/D22495