stabilize opt-level={s,z}
closes #35784
closes #47651
### Rationale
Since the lastest LLVM upgrade rustc / LLVM does more agressive loop unrolling. This results in increased binary size of embedded / no_std programs: a hundreds of bytes increase, or about a 7x increase, in the case of the smallest Cortex-M binary cf. #49260.
As we are shooting for embedded Rust on stable it would be great to also provide a way to optimize for size (which is pretty important for embedded applications that target resource constrained devices) on stable.
Also this has been baking in nightly for a long time.
r? @alexcrichton which team has to sign off this?
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| README.md | ||
For more information about how rustc works, see the rustc guide.