sim: add preliminary support for --enable-targets

This doesn't actually create one `run` program like other projects,
but creates multiple `run-$arch` targets.  While it might not seem
that useful initially, this has some nice properties:
- Allows us to quickly build all sim targets in a single tree.
- Positions us better for converting targets over to a proper
  multitarget build+install.

We don't have the ability to actually run tests against them, but
that's due to a limitation in gas: it doesn't support multitarget.
If that ever changes, we should be able to turn on our tests too.
We can improve the test framework to fallback to a system toolchain
if available to help mitigate that.

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