I think its handy to provide for a OBJDIRSUFFIX that may be independent
of MACHINE*.  I'm not so crazy about the symlink bit - perhaps there 
should be a knob to enable that?

FWIW you could do this sort of thing use MAKEOBJDIR in the environment:

MAKEOBJDIR='obj.${MACHINE_ARCH}-${OBJECT_FMT}'

note the single quotes - MACHINE_ARCH and OBJECT_FMT won't be resolved
until make goes to see if that dir exists.  This would allow the sparc
folk to just change MACHINE_ARCH in the environment, and not have to 
worry about the symlinks.

BTW you can do quite cool things with MAKEOBJDIR:

MAKEOBJDIR='${.CURDIR:S,${SRCTOP},${OBJTOP},}'

allows you to create an object tree under OBJTOP which parallels SRCTOP
in much the same way MAKEOBJDIRPREFIX works, but neater - eg. you get:
/var/obj/bin/cat
instead of:
/var/obj/u2/sjg/NetBSD/current/src/bin/cat

--sjg