Let the variable have a random name since we are not going to reference
it. This allows more than one WITH_SEMAPHORE() per context and also to
avoid shadowing if we open inner contexts.
We don't need the specialized one inside WithSemaphore, so use the
abstract one which allows us to more easily convert between current API
and the HAL_Semaphore one.
While at it also remove additional pragma and allow constructor to
receive a pointer for convenience (we will just dereference it).