Increase size of thread table.

Need larger thread table for X10.
Use HEURISTIC2 to find stackbottom on BlueGene systems

The Linux Compute Node Kernel used on BlueGene systems does not
support using LINUX_STACKBOTTOM to find the bottom of stacks.
Instead, use HEURISTIC2.

Verified to work with X10 on both BlueGene/P and BlueGene/Q systems.