rts: Allow building with ASSERTs on in non-DEBUG way

We have a couple of places where the conditions in asserts depend on code
ifdefed out when DEBUG is off. I'd like to allow compiling assertions into
non-DEBUG RTSen so that won't do.

Currently if we remove the conditional around the definition of ASSERT()
the build will not actually work due to a deadlock caused by initMutex not
initializing mutexes with PTHREAD_MUTEX_ERRORCHECK because DEBUG is off.