Move MACHINE/MACHINE_ARCH handling to sys.machine.mk

Rather than define the TARGETS and TARGET_ARCHES in src/Makefile
put them in sys.machine.mk so they can also be leveraged by
non-top-level builds.

We have TARGET_MACHINE_LIST as the list of MACHINES we build for.

For each MACHINE we have a MACHINE_ARCH_LIST_${MACHINE}
and the first entry of each list is used as default for
MACHINE_ARCH_${MACHINE}

We can now remove a bunch of MACHINE_ARCH.* handling from
local.sys.dirdeps*mk

Reviewed by: imp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D40083