Enable `--all-targets` for `x.py check` unconditionally

Now that Cargo deduplicates diagnostics from different targets, this doesn't flood the console with
duplicate errors.

Note that this doesn't add `--all-targets` in `Builder::cargo` directly because `impl Step for Std`
actually wants to omit `--all-targets` the first time while it's still building libtest.

When passed `--all-targets`, this warns that the option isn't needed, but still continues to compile.

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