ghc-toolchain: Toolchain Selection

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 used by Hadrian

  * But we additionally validate the Target files generated by
    ghc-toolchain against the ones generated by configure, to get a head
    start on catching configuration bugs before we transition
    completely.

  * When we make that transition, we will want to drop a lot of the
    toolchain configuration logic from configure, but keep it otherwise.

* For each compiler stage we should have 1 target file (up to a stage compiler we can't run in our machine)
  * We just have a HOST target file, which we use as the target for stage0
  * And a TARGET target file, which we use for stage1 (and later stages, if not cross compiling)
  * Note there is no BUILD target file, because we only support cross compilation where BUILD=HOST
  * (for more details on cross-compilation see discussion on !9263)

See also
* Note [How we configure the bundled windows toolchain]
* Note [ghc-toolchain consistency checking]
* Note [ghc-toolchain overview]

Ticket: #19877
MR: !9263