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.

Fix verified via execution of X10 with GC enabled (gc version 7.2d)
on both BlueGene/P and BlueGene/Q systems.