Add the bits required to build rust on NetBSD/powerpc ports, and
point to the bootstrap kit for NetBSD/powerpc I'm hosting at the
moment.

Also add the bits I used when cross-building the NetBSD/powerpc
rust on amd64, commented out, as well as the gcc / c++ wrapper
script I used in the process.

The changes affecting other ports are:
* We now add LD_LIBRARY_PATH in the make environment, so that
  if the bootstrap kit binaries and shared libraries don't have
  the $ORIGIN-style RPATH entries, it will still work
* The bootstrap.py script has been changed to turn off the
  generation of debuginfo in "RUSTFLAGS"; for some so far unknown
  reason, the NetBSD/powerpc rust will not build if you ask for
  debug info.  This could perhaps have been made OS-variant dependent,
  but isn't at the moment.

So .. bump PKGREVISION.

(he)