In CL 419915, both pointer fields of the PtraceIoDesc struct were
converted to type uintptr to address golang/go#54113.

However, that change was overzealous: the fix needed to convert fields
that refer to addresses in the child process, but the Addr field of
PtraceIoDesc is not even in the child process! It is instead an
address in the parent (Go) process.

Go's unsafe.Pointer rules prohibit converting a Go pointer to a
uintptr except when immediately converting back to an unsafe.Pointer
or calling a system call. Populating a PtraceIoDesc struct is neither
of those things, so converting the Addr field to uintptr introduced a
use-after-free bug.

This change reverts the change to the Addr field from CL 419915 and
consolidates the implementation of PtraceIO to reduce the the amount
of code that varies with GOARCH.

This change does not address the remaining ptrace uintptr bug
(golang/go#58387), which is also present in the Linux implementation.

Fixes golang/go#58351.
Updates golang/go#54113.
For golang/go#41205.

Change-Id: I14bdb4af42130aa7b4375e3f53fd1a0435f14307
Reviewed-on: https://go-review.googlesource.com/c/sys/+/465676
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>