Building RELENG_9 (or RELENG_9_*) on a small machine?

[Darren Pilgrim]

So I have this VM which only has 256 MB of memory and 9.1-R installed. 
I want to update it to RELENG_9, but buildworld swaps so bad it grinds 
nearly to a halt.  Even make -j1 -B runs into very heavy swapping.  So 
two questions:

1. Is there anything I can do to set a limit on how much memory the 
compiler uses?

2. If I crossbuild for this machine, can I get away with doing 
buildworld on a buildbox, ship the obj tree, and do installworld and 
mergemaster on the VM?  It's been a few enternities since I did a cross 
build and back then it was crossbuilding releases.