Fixes for BlueGene/P, BlueGene/Q, Power/Linux and FX10 systems from X10 project

As requested, changes made to 7.2 resubmitted against master.

These are some accumulated small patches we've been maintaining as part of the X10 project (http://http://x10-lang.org/). It would be great if we could get some/all of these merged back into the mainline. They enable BDWGC to be used on a variety of HPC Systems (IBM BlueGene/P and BlueGene/Q, Fujitsu's FX10, and on Power/Linux systems with IBM Parallel Environment).

We've tested them extensively on 7.2, but are not setup to easily test them against master.