Java doesn't trust pthread_getattr_np() to work for the main thread in
Linux, so instead it relies on a global variable __libc_stack_end, which
we didn't implement and therefore causing an annoying message.

This patch implements __libc_stack_end. Pardoxically, this shouldn't point
to the real stack end (we can easily find this our sched::thread interfaces)
but a little below it, because Java expects the address to be in the
stack, not one byte above it. So we use __builtin_frame_address(0) to
find the current frame's address.

Unfortunately, while this elliminates one warning message, another one
remains - because Java later expects to read /proc/self/maps and doesn't
find it.