Rodrigo Mesquita authored Jun 26, 2023 and Matthew Pickering committed Jul 24, 2023

This commit integrates ghc-toolchain, the brand new way of configuring
toolchains for GHC, with the Hadrian build system, with configure, and
extends and improves the first iteration of ghc-toolchain.

The general overview is

* We introduce a program invoked `ghc-toolchain --triple=...` which, when run,
  produces a file with a `Target`. A `GHC.Toolchain.Target.Target`
  describes the properties of a target and the toolchain (executables
  and configured flags) to produce code for that target

* Hadrian was modified to read Target files, and will both
  * Invoke the toolchain configured in the Target file as needed
  * Produce a `settings` file for GHC based on the Target file for that stage

* `./configure` will invoke ghc-toolchain to generate target files, but
  it will also generate target files based on the flags configure itself
  configured (through `.in` files that are substituted)

  * By default, the Targets generated by configure are still (for now) the ones ...