Move MACHINE/MACHINE_ARCH handling to sys.machine.mk

[sjg]

Summary

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

Sponsored by: Juniper Networks, Inc.

[sjg]

Keep ALL_MACHINE_LIST in local.sys.dirdeps.env.mk

dirdeps.mk etc works on ALL_MACHINE_LIST so it may end up different
to TARGET_MACHINE_LIST

[imp]

Seems ok.