riscv-macros: Move post_init and entry attributes

[romancardenas]

This PR is part of the rework for procedural macros of the RISC-V crates. Basically, it migrates riscv_rt_macros::post_init to riscv_macros::post_init.

The behavior of the macro remains mainly identical. However, I worked on a more modular implementation, with the idea of reusing as much code as possible once this PR is merged and we move entry, exception, etc. Hopefully, it will be easier in the future to maintain all the riscv-rt macros :)

I also modified the multi-hart QEMU example to illustrate how the macro works (and assert that it works properly).

Let me know what you think!

[rmsyn]

Thanks for adding this test case to the QEMU test suite.

Maybe add cases for positive/negative results for each of the check_* functions to the test-trybuild suite?

[romancardenas]

Sure! There are already a few, but I am also thinking on a new error handling mechanism for these macros that stores multiple errors and throws them in batch. In this way, developers will able to see all the errors with just one attempt

[romancardenas]

@rmsyn check it out, I think the new error handling is way better than in previous versions!

[romancardenas]

(of course Rust 1.93 came with new clippy lints and compile error messages that broke our CI...)

[romancardenas]

Now, entry is also in riscv-macros. I added trybuild tests to check that everything works as expected.

[romancardenas]

Closed as per #389