Backport stuff that never made it back to nachos-private

These changes were in the tarball distribution, but not in the master copy this
repo was generated from.  I've now backported them in here
Updates to the test scripts for building the mips cross-compiler.

The instructions on building a cross-compiler to use the latest binutils/gcc
have been updated and should work on any platform now.

This required some very slight tweaks to files in the nachos/test directory:
"script" and "va-mips.h".