Revert "Revert "runtime: consolidate function descriptor definitions on PPC64""

Revert "Revert "runtime: consolidate function descriptor definitions on PPC64""

This reverts CL 481059, which in turn reverts CL 478917.

Reason for revert: reapply the original CL.

Change-Id: Icf6bb6a620313b44fadcc7f69a62fdbb943e34fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/481075
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Paul Murphy <murp@ibm.com>