The sim_init function was called by runtest for each test when --tool
was set to sim. When we changed to --tool '' to collapse the testsuite
dir, the init function was no longer called on every test. However, it
was still being called explicitly by config/default.exp. It's not clear
why that explicit call ever existed since, in the past, it meant it was
redundant.
Lets drop the single sim_init call in config/default.exp and move it out
to all our tests. This replicates the runtest behavior so we can setup
variables on a per-test basis which allows us to recollapse the sim_path
logic back. We'll also leverage this in the future for toolchain setup.
Also add a few comments clarifying the overall runtime behavior.
These haven't been used in over 20 years. The sim testsuite used to
run these tools itself directly, but back in ~1999 it switched to the
dejagnu helpers (e.g. target_assemble & target_link), and the dejagnu
logic only utilizes XXX_FOR_TARGET variables. Punt them here to avoid
confusion with dead code.
