Teach `compiletest` to ignore platform triples
The UI tests are written assuming `--remap-path-prefix` is *not used* (`remap-debuginfo` in `config.toml`). The consequence is that the error messages may include paths and snippets into the standard library. When `remap-debuginfo` is enabled, these messages change in format and structure because `rustc` will not show paths and snippets into the standard library.
This normally isn't a problem for the "main" platforms (linux/macos/windows), because the CI infrastructure is set up so that the tests run without `remap-debuginfo`, but the `dist` artifacts are built separately with `remap-debuginfo` enabled. However, some of the lower-tier platforms perform both tests and distribution in a single step with `remap-debuginfo` enabled. This also affects developers and distributors who use `remap-debuginfo`.
To sidestep this problem, we add a way to ignore tests in specific platform triples, and update the overly broad `ignore-x86` rule in affected tests.
Address #46948, #54546, #53081.
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