cmd/vet: add vet runner script for core

cmd/vet: add vet runner script for core

This CL adds a script to run vet on std and cmd.

There are a considerable number of false positives,
mostly from legitimate but unusual assembly in
the runtime and reflect packages.

There are also a few false positives that need fixes.
They are noted as such in the whitelists;
they're not worth holding this CL for.

The unsafe pointer check is disabled.
The false positive rate is just too high to be worth it.

There is no cmd/dist/test integration yet.
The tentative plan is that we'll check the local platform
during all.bash, and that there'll be a fast builder that
checks all platforms (to cover platforms that can't exec).

Fixes #11041

Change-Id: Iee4ed32b05447888368ed86088e3ed3771f84442
Reviewed-on: https://go-review.googlesource.com/27811
Reviewed-by: Rob Pike <r@golang.org>