Patch for Native Client builds.

This patch contains the source changes necessary to compile Gromacs
for Native Client.  Patch is based on original work by Ivan Krasin,
additional changes from Joseph Coffland.
Also included are a few compiler warning fixes and a minor FAHCORE
tweak.

Change-Id: I085c52ff1d8e45ec8ffb8c56f5877313d6225bb2