cmd: add README generation for compiler + linker script tests
Add in automatic README generation and README consistency checking for
the cmd/compile and cmd/link script tests. This code is adapted from
the similar facility in cmd/go (e.g. scriptreadme_test.go); the README
helps folks writing new tests understand the mechanics.
Updates #68606.
Change-Id: I8ff7ff8e814abd4385bd670440511b2c60a4cef6
Reviewed-on: https://go-review.googlesource.com/c/go/+/601756 Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>