runtime: on AIX check isarchive before calling libpreinit

runtime: on AIX check isarchive before calling libpreinit

On AIX, all externally linked programs call _rt0_ppc64_aix_lib,
as seen in runtime/cgo/gcc_aix_ppc64.c. The idea is that we
only do the library initialization is isarchive is set.
However, before this CL, AIX programs would call libpreinit
before checking isarchive. The effect was that signals were
initialized twice. That caused the signal code to record that
all signals had an existing forwarding address, namely the
Go signal handler that was always installed. This caused signal
forwarding to enter an infinite loop. This caused, for example,
"go test os" to hang forever when running TestStdPipe.

Fixes #76081

Change-Id: I5555f8c5e299d45549f5ce601568dc6b3cff4ecc
Reviewed-on: https://go-review.googlesource.com/c/go/+/727820
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>