Rollup merge of #77441 - couchand:2020-10/fix-75504, r=jonas-schievink

Fix AVR stack corruption bug

Updates the Rust LLVM fork to pull in a fix for a stack corruption bug in AVR platform interrupt code lowering.

Fixes #75504

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It looks like this is also pulling in a register selection fix by @Amanieu and @tambry that hasn't merged yet; I don't see an open PR for that update.

Author: jonas-schievink