V4 Use getrlimit on Darwin to get the stack size for the main thread

Turns out pthread_get_size does not return the correct size when it is
called from the main thread, so to workaround you call getrlimit instead
Without this change, most QML applications are broken on iOS.

Change-Id: I9a61494de26caa3d7be7e46a991e6d6d0514ce17
Reviewed-by: Lars Knoll <lars.knoll@digia.com>