Fix some PPC ABI issues.

- Use 16 byte alignment for stack pointer.

- Initialize backchain pointer in the stack frame when calling out from
  generated code.  This commit simply initializes it to zero (to avoid
  crashing in OS::StackWalk).  In the future, we should consider
  recording the stack pointer of the function which called into the
  generated code and initialize the backchain with that.

- Cleanup/simplify non-volatile register usage.