Consider moving our own RISCV RT overrides to its own crate

[bjoernQ]

Currently the HALs targeting RISCV (ESP32-C2 and ESP32-C3 currently) include some replacements for the assembly found in riscv-rt

That's redundant code in both HALs - in future we will see more RISCV based chips so it makes sense to move that common functionality into its own crate

While on it we should consider if we want to remove the dependency to riscv-rt completely since there is very little, we use from that

Note: While initially we did that because the mentioned targets don't support mie and ``mip` CSRs we now also need special versions of the trap handler - that means we will continue to need it

[bjoernQ]

Also, we could re-export riscv from there and probably can solve situations like the recent yanking of a bunch of riscv versions by releasing a minor update to our crate

[jessebraham]

I started working on this awhile back actually, I'll dust off the project and try to get it in a repo this week some time.

[jessebraham]

The package has yet to be published, but a repository has been created:
https://github.com/esp-rs/esp-riscv-rt

Once we've tested it a bit more I will get a release out and update the necessary dependencies to use this package instead.

[jessebraham]

Closed by #365