sim: testsuite: drop most specific istarget checks

We'll rely on the toolchain probing to determine whether each arch's
tests can be run rather the current configure target.  This allows
testing all of the ports in a multitarget configuration.

For now, we don't reformat the files entirely to make it easier to
review, and in case we need to make adjustments.  Once this feels
like it's stable, we can flatten the code a bit by removing the if
statement entirely.