syscall: introduce IoctlPtr for exec_unix tests

syscall: introduce IoctlPtr for exec_unix tests

Avoid passing Go pointers as uintptr in exec_unix_test.go by introducing
syscall.IoctlPtr() which accepts arg as unsafe.Pointer.

For #44834
Fixes #58609

Change-Id: I6d0ded023e5f3c9989783aee7075bb88100d9ec2
Reviewed-on: https://go-review.googlesource.com/c/go/+/469675
Run-TryBot: Dmitri Goutnik <dgoutnik@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>