runtime: on AIX, fix call to _cgo_sys_thread_create in _rt0_ppc64_aix_lib

The AIX ABI requires allocating parameter save space when calling
a function, even if the arguments are passed via registers.

gcc sometimes uses this space. In the case of the cgo c-archive
tests, it clobbered the storage space of argc/argv which prevented
the test program from running the expected test.

Fixes #68957

Change-Id: I8a267b463b1abb2b37ac85231f6c328f406b7515
Reviewed-on: https://go-review.googlesource.com/c/go/+/606895


Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>