runtime: fix _rt0_ppc64x_lib on aix

runtime: fix _rt0_ppc64x_lib on aix

CL 706395 refactored the ppc64 library entry point and missed some
important aix-specific characteristics:

- _rt0_ppc64x_lib should account for the function descriptor when
getting the callback pointer.

- _rt0_ppc64x_lib should only call _cgo_sys_thread_create when
built as a c-archive.

Fixes #75801

Cq-Include-Trybots: luci.golang.try:gotip-linux-ppc64_power10
Change-Id: I343ca09d3b9688ffa585668a6c52f0ad519d6203
Reviewed-on: https://go-review.googlesource.com/c/go/+/710175
Reviewed-by: Paul Murphy <paumurph@redhat.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>