ghc821-binary: use glibc on musl

These prebuilt binaries need to be used with glibc,
it doesn't matter what we're using otherwise.

This may seem a bit strange but has the huge advantage
of not needing to create/host/download musl-specific variants,
and instead continue to use the official ones from upstream.

pkgs/development/compilers/ghc/8.2.1-binary.nix

@@ -1,6 +1,6 @@
 { stdenv
 , fetchurl, perl, gcc, llvm_39
-, ncurses5, gmp, libiconv
+, ncurses5, gmp, glibc, libiconv
 }:
 
 # Prebuilt only does native
@@ -14,6 +14,12 @@ let
   libEnvVar = stdenv.lib.optionalString stdenv.hostPlatform.isDarwin "DY"
     + "LD_LIBRARY_PATH";
 
+  glibcDynLinker = assert stdenv.isLinux;
+    if stdenv.hostPlatform.libc == "glibc" then
+       stdenv.cc.bintools.dynamicLinker
+    else
+      "${stdenv.lib.getLib glibc}/lib/ld-linux*";
+
 in
 
 stdenv.mkDerivation rec {
@@ -95,7 +101,7 @@ stdenv.mkDerivation rec {
       find . -type f -perm -0100 -exec patchelf \
           --replace-needed libncurses${stdenv.lib.optionalString stdenv.is64bit "w"}.so.5 libncurses.so \
           --replace-needed libtinfo.so libtinfo.so.5 \
-          --interpreter "$(cat $NIX_CC/nix-support/dynamic-linker)" {} \;
+          --interpreter ${glibcDynLinker} {} \;
 
       paxmark m ./ghc-${version}/ghc/stage2/build/tmp/ghc-stage2