[dev.typeparams] cmd/compile: add stenciling of simple generic functions

Allow full compilation and running of simple programs with generic
functions by stenciling on the fly the needed generic functions. Deal
with some simple derived types based on type params.

Include a few new typeparam tests min.go and add.go which involve
fully compiling and running simple generic code.

Change-Id: Ifc2a64ecacdbd860faaeee800e2ef49ffef9df5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/289630


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>