Break up the dejagnu logic so that we can parallelize the testsuite.
This takes a page from gcc & gdb where each .exp is run in isolation
instead of in serial.
For most targets, this doesn't make much of a difference as they only
have a single .exp. A few (like cris & frv) have multiple .exp though
and will see a bit of a speed up.
The real gain is when testing a multitarget build. This way we can
run all the targets in parallel and cut the execution time a bit.
On my system, it goes from ~155sec to ~100sec.
We can gain further speedups by splitting up some of the larger .exp
files into smaller groups. We'll do that in a followup though.
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