Auto merge of #2606 - cbiffle:nostdio, r=RalfJung

Magic functions for writing to stdout/stderr.

This enables I/O in no_std contexts (or, really, any Miri-specific OS-independent context). Combined with the `abort` intrinsic it should allow a reasonable test framework in no_std.

**Question for maintainers:** So, the `no_std` panic test needs work, for two reasons:

- First, its stdout includes Miri's whole message about the abort intrinsic having been used. I guess whatever panic handler you use in `std` contexts exits cleanly without triggering this message. Comparing the entire output with backtrace as golden seems fragile.
- Second, likely for the same reason, the test framework appears to expect the test to exit successfully, when in fact it exits with status 1 due to the abort. This means the test doesn't actually pass right now.

What shall I do there?

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