segger: config: RTT spinlock implementation

RTT locking is now implemented with spinlocks:
* The prior implementation did not support RTT logging and tracing from
  ISR context.
* Mutexes caused infinite loops when tracing.
* Spinlocks are more lightweight than mutexes, especially on non-SMP
  systems.

Fixes: #61133

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>