Add --cap-lints option

> rustc --help
...
        --cap-lints LEVEL
                        Set the most restrictive lint level. More restrictive
                        lints are capped at this level
...

I believe that this option allows the compilation of crates while ignoring some errors, for backward compatibility. More information about the option can be found in the RFC introducing it.

I think the paragraph that resumes the behavior of the option the best is the following:

For example when --cap-lints allow is passed, all instances of #[warn], #[deny], and #[forbid] are ignored. If, however --cap-lints warn is passed only deny and forbid directives are ignored.
The acceptable values for LEVEL will be allow, warn, deny, or forbid.

Using something like -frust-cap-lints=LEVEL would also probably make sense and be easy to use. I think this also relies a bit on macros being implemented in order to handle different level of lints directly in the source using the aforementioned directives

Start with the RFC introducing cap-lints, linked in the issue, and compare its semantics with the reported rustc --help text. Trace how gccrs handles compiler options and lint levels; the work is done when allow, warn, deny, and forbid are accepted and the documented capping behavior is supported.