runtime, runtime/cgo: conform to PIC register use rules in ppc64 asm

PIC code on ppc64le uses R2 as a TOC pointer and when calling a function
through a function pointer must ensure the function pointer is in R12.  These
rules are easy enough to follow unconditionally in our assembly, so do that.

Change-Id: Icfc4e47ae5dfbe15f581cbdd785cdeed6e40bc32
Reviewed-on: https://go-review.googlesource.com/15526


Reviewed-by: Ian Lance Taylor <iant@golang.org>