The `io/run` script will run the io interpreter on a step file given in
the `$STEP` environment variable. It'll swallow the first 25 bytes
printed by the interpreter because they are a startup message about the
Io Regex module (and can't be suppressed in a normal way).
- Remove most of the step5 excludes in the Makefile except for ones
which don't have TCO capability at all (or the implementation is too
slow): bash, make, mal, matlab.
- Make perf_EXCLUDES consistent with other excludes.
- Add a print-FOO target which prints the resolved value of Makefile
variable FOO. For example, `make print-IMPLS` to print the list of
implementations.
