Auto merge of #33363 - japaric:target, r=japaric

fix built-in target detection

previously the logic was accepting wrong triples (like
`x86_64_unknown-linux-musl`) as valid ones (like `x86_64-unknown-linux-musl`) if
they contained an underscore instead of a dash.

fixes #33329

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r? @brson

I wanted to use a compile-fail test at first. But, you can't pass an extra `--target` flag to `rustc` for those because they already call `rustc --target $HOST` so you get a `error: Option 'target' given more than once.`. The run-make test used here works fine though.

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