[release-branch.go1.12] runtime: safely load DLLs

While many other call sites have been moved to using the proper
higher-level system loading, these areas were left out. This prevents
DLL directory injection attacks. This includes both the runtime load
calls (using LoadLibrary prior) and the implicitly linked ones via
cgo_import_dynamic, which we move to our LoadLibraryEx. The goal is to
only loosely load kernel32.dll and strictly load all others.

Meanwhile we make sure that we never fallback to insecure loading on
older or unpatched systems.

This is CVE-2019-9634.

Fixes #30666
Updates #14959
Updates #28978
Updates #30642

Change-Id: I401a13ed8db248ab1bb5039bf2d31915cac72b93
Reviewed-on: https://go-review.googlesource.com/c/go/+/165798


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
(cherry picked from commit 9b6e9f0c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/168339


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>