[riot-notifications] [RIOT-OS/RIOT] core/mutex: Minor cleanup (#15462)

[Marian Buschsieweke]

### Contribution description

- Split out handling of the blocking code path of mutex_lock() into a static  `_block()` function. This improves readability a bit and will ease review of  a follow up PR.
- Return `void` instead of `int`.
- Made debug output more consistent

### Testing procedure

All the mutex test applications (`cpp11_mutex`, `mutex_order`, `mutex_unlock_and_sleep`, `rmutex`, `rmutex_cpp`, `xtimer_mutex_lock_timeout`, `xtimer_rmutex_lock_timeout`) should still work and `bench_mutex_pingpong` should still yield the same performance.


### Issues/PRs references

Split out of https://github.com/RIOT-OS/RIOT/pull/15442, which was split out of https://github.com/RIOT-OS/RIOT/pull/15052
You can view, comment on, or merge this pull request online at:

  https://github.com/RIOT-OS/RIOT/pull/15462

-- Commit Summary --

  * core/mutex: consistent debug output
  * core/mutex: clean up mutex_lock()

-- File Changes --

    M core/include/mutex.h (4)
    M core/mutex.c (68)

-- Patch Links --

https://github.com/RIOT-OS/RIOT/pull/15462.patch
https://github.com/RIOT-OS/RIOT/pull/15462.diff

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