Forward declare __sev(), __wfe(), __wfi()

[armandomontanez]

If the compiler in use offers __sev(), __wfe(), or __wfi(), forward declare them to be sure they're available for use.

Recent LLVM/Clang changes are enforcing #include <arm_acle.h> as the way to get this builtin:

external/pico-sdk+/src/common/pico_sync/mutex.c:77:9: error: call to undeclared library function '__wfe' with type 'void (void)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   77 |         lock_internal_spin_unlock_with_wait(&mtx->core, save);
      |         ^
external/pico-sdk+/src/common/pico_sync/include/pico/lock_core.h:128:95: note: expanded from macro 'lock_internal_spin_unlock_with_wait'
  128 | #define lock_internal_spin_unlock_with_wait(lock, save) spin_unlock((lock)->spin_lock, save), __wfe()
      |                                                                                               ^
external/pico-sdk+/src/common/pico_sync/mutex.c:77:9: note: include the header <arm_acle.h> or explicitly provide a declaration for '__wfe'

Forward declaring using the same signature as arm_acle.h makes clang happy again.