defmt-semihosting: use critical_section for synchronization.

[driveraid]

This change allows defmt-semihosting to be used in multi-core contexts. It updates synchronization to match that of defmt-rtt.

[jonathanpallant]

Thank you for the PR - I agree this kind of change needs doing.

Unfortunately defmt_rtt's approach here isn't perfect. Instead of a static mut there should probably be an UnsafeCell and instead of three variables, there should probably be one static struct Context to keep it all tidy. I also want to switch to the semihosting crate, because it works on more architectures than just Cortex-M.

[driveraid]

Do you want all of those in the same PR? Or is it perhaps better in the interest of keeping single-purpose PRs to let this PR merge and cover the other requests in follow-up PRs?

[driveraid]

... I mean, since there's some refactoring to do (update defmt-rtt to do the same etc)

[jonathanpallant]

This is great as-is, so LGTM.

[driveraid]

Draft PR up for one struct Context: #944