internal/syscall/windows: use unsafe.Pointer instead of uintptr

internal/syscall/windows: use unsafe.Pointer instead of uintptr

Some functions accept a uintptr when they should accept an
unsafe.Pointer, else the compiler won't know that the pointer should
be kept alive across the call, potentially causing undefined behavior.

Fixes #73156 (potentially)

Change-Id: I29c847eb8ffbb785fabf217e9f3718d10cfb5047
Reviewed-on: https://go-review.googlesource.com/c/go/+/662855
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>