Auto merge of #141210 - RalfJung:miri-std-doctests, r=saethlin

tools-aux ci runner: also cross-test doctests in Miri

Miri now supports running doctests across different targets. Let's use that to run the std doctests on aarch64-apple-darwin, i686-pc-windows-msvc.

try-job: x86_64-gnu-aux

Author: bors