speed up MallocExtension::instance()

It was reported that pthread_once is expensive, especially on ppc.

In new implementation in hot path instead of doing potentially
expensive atomic read with barrier, we do just plain read.

It's slightly less robust than older implementation, but it should be
faster.

New code is making assumption that programs do not spawn threads
before main() is called. And therefore all variables & modules are
initialized before threads are created. Which looks like pretty safe
assumption. With that assumption, doing plain read is safe, because
current_instance is initialized as part of module init and therefore
before threads are spawned.

This patch is based on feedback of Adhemerval Zanella.