internal/syscall/unix: change Ioctl arg type to unsafe.Pointer on AIX

Without this change, this code is technically in violation of the
unsafe.Pointer rules since the conversion from unsafe.Pointer to uintptr has
to happen when calling into the syscall6 assembly implementation.

Change-Id: I4821f5bf9788c8fa2efeb041f811ed092e07ae74
Reviewed-on: https://go-review.googlesource.com/c/go/+/340949


Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>