misc/cgo/test: reduce likeliness of hang in Test9400

If a GC triggers while spinning in RewindAndSetgid, it may result in
this test hanging. Avoid it by disabling the collector before entering
the uninterruptable ASM conditional wait.

Fixes #49695

Change-Id: Ie0a03653481fb746f862469361b7840f4bfa8b67
Reviewed-on: https://go-review.googlesource.com/c/go/+/365836


Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>