Possibilities with new altjit naming convention

[kunalspathak]

From @AndyAyersMS in #282 (comment)

Now that the altjit naming is systematic I wonder if we should add "wildcard" support, eg run batches of diffs using all known altjit os/isa combinations or something?

Also if/when a regular jit can run as an altjit some of the file swapping done by jit-dasm-pmi can be simplified as we no longer need to replace the "real jit" -- and once we do this we can perhaps also change the concurrently model for jit-dasm-pmi to be more like the one we use for crossgen (run base and diff tasks concurrently).