Update to the latest version of mrustc

[emosenkis]

Verify that build from scratch and update from current version of mrustc both work correctly.

[emosenkis]

Updated. This will need to be done again periodically.

[thepowersgang]

Needs to be done again, as the target revision fails under some configurations (likely due to UB) - see thepowersgang/mrustc#74

[emosenkis]

@thepowersgang updated.

[spott]

Is there a reason that this pins mrustc to a specific commit? If there is, it would be nice to be documented.

The version of mrustc that is pinned at the moment doesn't support MacOS, so I've got to find a different version of mrustc to use, and I don't know how to make that decision.

[emosenkis]

@spott there's nothing special about the pinned mrustc commit, it's just a version that's known to build and run successfully using the commands in this script.

[spott]

Thanks. I'll have to try and find a version that compiles for OS X.