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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: !926331dcd26c
