Add `cfg revisions` in `gccrs` testsuite

[Revisions](https://rustc-dev-guide.rust-lang.org/tests/compiletest.html#revisions) allow a single test file to be used for multiple tests. This is done by adding a special header at the top of the file:

```
//@ revisions: foo bar baz 
```

Reference: https://rustc-dev-guide.rust-lang.org/tests/ui.html#cfg-revisions