sim: testsuite: fix objdir handling

The tests assume that the cwd is the objdir directory and write its
intermediates to there all the time.  When using runtest's --objdir
setting though, this puts the files in the wrong place.  This isn't
a big problem currently as we never change --objdir, but in order to
support parallel test execution, we're going to start setting that
option, so clean up the code ahead of time.

We also have to tweak some of the cris tests which were making
assumptions about the argv[0] value.