Per discussions over how to lighten the load of armv6, move it to an
extra arch. You can still build TARGET_ARCH=armv6 if you desire, but
it won't be built as part of 'make universe' without -DEXTRA_TARGETS.
Sponsored by: Netflix
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