glibc: fix cross-compile to ppc64le

Fixes cross-compilation when build == host != target == ppc64le.
Glibc invokes objcopy during cross-compilation to ppc64le, which
fails when the nonprefixed objcopy can't understand the target format.

pkgs/development/libraries/glibc/common.nix

@@ -160,6 +160,10 @@ stdenv.mkDerivation ({
       "libc_cv_as_needed=no"
     ] ++ lib.optional withGd "--with-gd";
 
+  makeFlags = [
+    "OBJCOPY=${stdenv.cc.targetPrefix}objcopy"
+  ];
+
   installFlags = [ "sysconfdir=$(out)/etc" ];
 
   outputs = [ "out" "bin" "dev" "static" ];