Get the Linux ptrace test working on PowerPC64

The test refers to user_regs_struct.rip so it can only ever have worked
on x86-64. Put this code inside an appropriate #if, and add a similar
case for PowerPC64. (If we do likewise for ARM we can probably remove
the XFAILs, but I have no way of testing that.)

Those changes are enough to get the test working for me on big-endian
PowerPC64 Fedora 19.

Differential Revision: http://reviews.llvm.org/D6108