Multi-image build process causes problems with AIX linker (librvm.a not found)

This is a long standing minor annoyance with the way we build the bootimage runner on AIX. The AIX linker/loader are much less permissive than they corresponding tools on Linux. In particular, the absolute path to dependent .so/.a files is remembered and the file is looked for at that absolute path when the executable linked against the .a is run.

This doesn't play nicely with out habit of building the .a files in target and then copying them to dist. The nightly sanity runs on AIX are ok, because it builds & runs each image in turn. Using buildit to run multi-config tests causes failures because it builds all the images, then runs all the tests. Only the last image built actually works (because the librvm.a used to build the first n-1 images has been blown away).