Update meta.sys.mk and related local*mk

Move the setting of TARGET_SPEC_VARS to local.sys.env.mk
so meta.sys.mk can do the processing, and include local.meta.sys.mk
later.

Move the setting of GENDIRDEPS_FILTER*_VARS from local.gendirdeps.mk
to local.meta.sys.mk so we can automatically set DEP_* at level 1+
to avoid syntax errors when DEP_* variables are used in conditionals
in Makefile.depend files.

Update gendirdeps.mk just to get the documentation about the above.
No functional change.

local.dirdeps.mk be more careful about adding to DIRDEPS to avoid
unnecessary overhead, and introducing cycles in the graph.
Also set DEP_MACHINE_CPUARCH.

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39737