QNX-specific implementation of the stack base retrieval.

The garbage collector needs the top of the stack.
On QNX, every thread has the TLS structure at the top
of the stack, then the stack extends below this TLS structure
This is the easiest and fastest way resolve the stack base
of the current thread.

Change-Id: I9cc9592928298d06aeaee0f6110d61bf3cf68e49
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>