Shared libraries on ppc64le will require a larger minimum stack frame (because
the ABI mandates that the TOC pointer is available at 24(R1)). Part 2a of
preparing for that is to have all bits of arch-independent and ppc64-specific
codegen that need to know call a function to find out.

Change-Id: I55899f73037e92227813c491049a3bd6f30bd41f
Reviewed-on: https://go-review.googlesource.com/15524Reviewed-by: Ian Lance Taylor <iant@golang.org>