Split ghc-toolchain executable to new packge

In light of #23690, we split the ghc-toolchain executable out of the
library package to be able to ship it in the bindist using Hadrian.

Ideally, we eventually revert this commit.

m4/ghc_toolchain.m4

@@ -47,7 +47,7 @@ AC_DEFUN([FIND_GHC_TOOLCHAIN],
         -ilibraries/ghc-platform/src -iutils/ghc-toolchain/src \
         -XNoImplicitPrelude \
         -odir actmp-ghc-toolchain -hidir actmp-ghc-toolchain \
-        utils/ghc-toolchain/Main.hs -o acghc-toolchain
+        utils/ghc-toolchain/exe/Main.hs -o acghc-toolchain
 
     # (1) Configure a toolchain for the build and host platform (we require that BUILD=HOST, so one toolchain suffices)
     rm -f acargs

utils/ghc-toolchain/exe/ghc-toolchain-bin.cabal

+cabal-version:      2.4
+name:               ghc-toolchain-bin
+version:            0.1.0.0
+synopsis:           Utility for managing GHC target toolchains
+description:
+bug-reports:        https://gitlab.haskell.org/ghc/ghc/-/issues
+author:             Ben Gamari
+maintainer:         ben@well-typed.com
+copyright:          (c) The GHC Developers
+
+executable ghc-toolchain-bin
+    main-is:          Main.hs
+    ghc-options:      -Wall
+    default-extensions: NoImplicitPrelude
+    build-depends:    base,
+                      directory,
+                      filepath,
+                      process,
+                      transformers,
+                      ghc-platform,
+                      ghc-toolchain
+    default-language: Haskell2010

utils/ghc-toolchain/ghc-toolchain.cabal

@@ -40,16 +40,3 @@ library
                       ghc-platform
     hs-source-dirs:   src
     default-language: Haskell2010
-
-executable ghc-toolchain
-    main-is:          Main.hs
-    ghc-options:      -Wall
-    default-extensions: NoImplicitPrelude
-    build-depends:    base,
-                      directory,
-                      filepath,
-                      process,
-                      transformers,
-                      ghc-platform,
-                      ghc-toolchain
-    default-language: Haskell2010