Add mutex and shared_mutex headers back

[salkinium]

This reverts parts of 4d604ff and adds back only the <mutex> and <shared_mutex> headers back and their dependencies.
This allows me to use the lock guard code with custom types.

[ckormanyos]

Hi @salkinium nice work. I will need to update the readme accordingly, which kind of says we have no atomics and no thread support. The docs are now inconsistent with the impl.

I'll make a draft ASAP for your perusal.

Cc: @chris-durand

[salkinium]

Yeah, I thought about updating the docs, but the two PRs are not really functional. Atomics support requires external __atomic_* implementations that are in modm. And the useful threading support consists of only a few template functions. The mutex and shared_mutex code isn't even compiled due to the macros.

At least for the atomics I could add the generated AVR version of the __atomic_* hooks, that would make this actually useful.

[ckormanyos]

thought about updating the docs, but the two PRs are not really functional.

OK I didn't really know based on the scope of changes. My only real concern (and this seems to not be the case) is that the existing docs are actually wrong.

If there is any added value in making them more right, then OK. But that's optional.

Thank you @salkinium