A directory should only contain either library code, or contain a single `main`.

Attempting to have two main packages upsets the go tools.

Add a `run.sh` bash script in the root regres folder to `go run` the `cmd/regres/main.go` file, to keep usage simple for non-go people.

Change-Id: Ia37ce9a64e1e1a7e23ee3336e6e312cef9e63a9f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40528
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>