core/rmutex: use atomic utils
Replace use of C11 atomics with atomic utils. This fixes > error: address argument to atomic operation must be a pointer to a > trivially-copyable type ('_Atomic(int) *' invalid) error when compiling on AVR with LLVM.
Co-authored-by: Francisco Molina <femolina@uc.cl> Co-authored-by: Marian Buschsieweke <maribu@users.noreply.github.com>