[release-branch.go1.12-security] crypto/x509: mitigate CVE-2020-0601 verification bypass on Windows

An attacker can trick the Windows system verifier to use a poisoned set
of elliptic curve parameters for a trusted root, allowing it to generate
spoofed signatures. When this happens, the returned chain will present
the unmodified original root, so the actual signatures won't verify (as
they are invalid for the correct parameters). Simply double check them
as a safety measure and mitigation.

Windows users should still install the system security patch ASAP.

This is the same mitigation adopted by Chromium:

https://chromium-review.googlesource.com/c/chromium/src/+/1994434

Change-Id: I2c734f6fb2cb51d906c7fd77034318ffeeb3e146
Reviewed-on: https://go-review.googlesource.com/c/go/+/215905


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ryan Sleevi <sleevi@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/647124


Reviewed-by: Filippo Valsorda <valsorda@google.com>