Applications supplying their own custom stack to pthread_create are not
allowed to free the allocated memory after pthread_join returns as,
according to the specification, the thread is not guaranteed to be dead
after the join call returns.

Avoid this class of problems by avoiding the use of a custom stack
altogether, let pthread handle its own resources.

Allocations made on the child stack are now done on the C heap.

Thanks @semarie for noticing the problem on OpenBSD and suggesting a
fix.

Closes #7275