runtime: consolidate function descriptor definitions on PPC64
This is a cleanup to allow a consistent definitions of a function
descriptor on code shared between AIX and Linux. They need to be
declared in slightly different ways, but we can hide that in one
macro.
And, update all usage.
Change-Id: I10f3580473db555b4fb4d2597b856f3a67d01a53
Reviewed-on: https://go-review.googlesource.com/c/go/+/478917 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>