Use critical_section crate.

[reitermarkus]

Switch to using critical_section crate.

[katyo]

Actual versions of riscv crate isn't compatible with this crate, bacause &mut CriticalSection argument was removed from closure.
This PR solves it by removing potentially incompatible dependencies (cortex-m, riscv crates).

[Disasm]

Hi! Could you rebase your PR and add a changelog entry?

[reitermarkus]

@Disasm, PR is rebased.

[reitermarkus]

@Disasm, I fixed the CI matrix.

[Disasm]

Thank you!