haskellPackages: allow configuring gmp when cross compiling (#481162)

pkgs/development/compilers/ghc/common-hadrian.nix

@@ -50,8 +50,9 @@
   # If enabled, GHC will be built with the GPL-free but slightly slower native
   # bignum backend instead of the faster but GPLed gmp backend.
   enableNativeBignum ?
-    !(lib.meta.availableOn stdenv.hostPlatform gmp && lib.meta.availableOn stdenv.targetPlatform gmp)
-    || stdenv.targetPlatform.isGhcjs,
+    stdenv.targetPlatform.isGhcjs
+    || !(lib.meta.availableOn stdenv.hostPlatform gmp)
+    || !(lib.meta.availableOn stdenv.targetPlatform gmp),
   gmp,
 
   # If enabled, use -fPIC when compiling static libs.
@@ -668,7 +669,7 @@ stdenv.mkDerivation (
       "--with-ffi-includes=${targetLibs.libffi.dev}/include"
       "--with-ffi-libraries=${targetLibs.libffi.out}/lib"
     ]
-    ++ lib.optionals (targetPlatform == hostPlatform && !enableNativeBignum) [
+    ++ lib.optionals (!enableNativeBignum) [
       "--with-gmp-includes=${targetLibs.gmp.dev}/include"
       "--with-gmp-libraries=${targetLibs.gmp.out}/lib"
     ]