Add support for ppc64.
Note (1): This patch was rejected upstream because they don't have
appropriate hardware for testing.
Note (2): Upstream powerpc directory has some support for ppc64, but
only for Macs, and I couldn't get it to work at all with IBM hardware.
This patch was collaborated on by several people, most notably
David Woodhouse.
Includes fix for position of stack arguments to external C functions
when there are more than 8 parameters (RHBZ#829187).
Includes fix for minor heap corruption because of unaligned minor heap
register (RHBZ#826649).
Includes updates for OCaml 4.01.0.