Rewrite crunchgen target handling for progs so that it can be parallelized.

This covers 'clean', 'cleandepend', 'cleandir', 'obj', 'objlink' and
'build-tools'.

This uses the same method as bsd.subdir.mk.

MFC after: 2 weeks
X-MFC-With: r289731
Sponsored by: EMC / Isilon Storage Division