Our old redirectors were emitting code to call their target functions normally.
However, the PPC ABI expects callers to set up parameter save areas for their
callees, notably if the target is a varargs function.

Instead, mimic the pattern used when calling an external function or function
pointer and avoid touching the stack.

Change-Id: Ia28c9d2b82fcd99c4a2f70f5f587d0e0463a6f0e
Reviewed-on: https://boringssl-review.googlesource.com/17284
Reviewed-by: Adam Langley <agl@google.com>