os/exec: create extra threads when starting a subprocess
os/exec: create extra threads when starting a subprocess

TestExtraFiles seems to be flaky on GNU/Linux systems when using cgo
because creating a new thread will call malloc which can create a new
arena which can open a file to see how many processors there are.
Try to avoid the flake by creating several new threads at process
startup time.

For #25628

Change-Id: Ie781acdbba475d993c39782fe172cf7f29a05b24
Reviewed-on: https://go-review.googlesource.com/c/go/+/228099
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

akutz/go,akutz/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,akutz/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,akutz/go,golang/go,golang/go,cloudflare/go,akutz/go,nawawi/go,golang/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,golang/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go