NO_TLS used to be hardcoded on mac because up to 10.6, __thread was not
supported. Until recently, we still supported for 10.6, and it's not the
case anymore, so we could make mac builds use __thread.

Unfortunately, on OSX, __thread circles back calling malloc to allocate
storage on first access, so we have an infinite loop problem here.
Fortunately, pthread_keys don't have this property, so we can use that
instead. It doesn't appear to have significantly more overhead (and TLS
overhead is small anyways compared to the amount of work involved in
allocating memory with mozjemalloc).

At the same time, we uniformize the initialization sequence between
mozjemalloc and mozjemalloc+replace-malloc, such that we have less
occasions for surprises when riding the trains (replace-malloc being
nightly only), ensuring the zone registration happens at the end of
mozjemalloc's initialization.