pkgsMuslhaskell.compiler.{ghc884,ghc8104}: Use GHC 8.10 as bootstrap compiler.

This addresses the fact that `ghc865Binary` segfaults on musl
(see #118731) because of the glibc+musl mix used in there.

With the previous commits, `ghc8102Binary` was changed to use
the musl-based bindist from GHC HQ instead, which works.

With this change, all nix Haskell compilers builds on musl:

    NIX_PATH=nixpkgs=. nix-build --no-link --expr 'with import <nixpkgs> {}; { inherit (pkgsMusl.haskell.compiler) ghc884 ghc8104 ghc901 ghcHEAD; }'

pkgs/development/compilers/ghc/8.8.4.nix

@@ -142,11 +142,14 @@ stdenv.mkDerivation (rec {
   postPatch = "patchShebangs .";
 
   # GHC is a bit confused on its cross terminology.
-  preConfigure = lib.optionalString stdenv.isAarch64 ''
+  preConfigure =
     # Aarch64 allow backward bootstrapping since earlier versions are unstable.
-    find . -name \*\.cabal\* -exec sed -i -e 's/\(base.*\)4.14/\14.16/' {} \; \
-      -exec sed -i -e 's/\(prim.*\)0.6/\10.8/' {} \;
-  '' + ''
+    # Same for musl, as earlier versions do not provide a musl bindist for bootstrapping.
+    lib.optionalString (stdenv.isAarch64 || stdenv.hostPlatform.isMusl) ''
+      find . -name \*\.cabal\* -exec sed -i -e 's/\(base.*\)4.14/\14.16/' {} \; \
+        -exec sed -i -e 's/\(prim.*\)0.6/\10.8/' {} \;
+    ''
+  + ''
     for env in $(env | grep '^TARGET_' | sed -E 's|\+?=.*||'); do
       export "''${env#TARGET_}=''${!env}"
     done

pkgs/top-level/haskell-packages.nix

@@ -57,7 +57,8 @@ in {
 
     ghc884 = callPackage ../development/compilers/ghc/8.8.4.nix {
       # aarch64 ghc865Binary gets SEGVs due to haskell#15449 or similar
-      bootPkgs = if stdenv.isAarch64 then
+      # Musl bindists do not exist for ghc 8.6.5, so we use 8.10.* for them
+      bootPkgs = if stdenv.isAarch64 || stdenv.targetPlatform.isMusl then
           packages.ghc8102BinaryMinimal
         else
           packages.ghc865Binary;
@@ -67,7 +68,8 @@ in {
     };
     ghc8104 = callPackage ../development/compilers/ghc/8.10.4.nix {
       # aarch64 ghc865Binary gets SEGVs due to haskell#15449 or similar
-      bootPkgs = if stdenv.isAarch64 || stdenv.isAarch32 then
+      # Musl bindists do not exist for ghc 8.6.5, so we use 8.10.* for them
+      bootPkgs = if stdenv.isAarch64 || stdenv.isAarch32 || stdenv.targetPlatform.isMusl then
           packages.ghc8102BinaryMinimal
         else
           packages.ghc865Binary;