libiberty, libbfd: Make hash less fickle

These shouldn't respond to targetPlatform, but previously did. The
reason is somewhat complex: they would rely on the sources of gcc and
binutils, respectively, which *do* depend on the target platform.
Obviously the source is the same in all cases, but when those packages
are no longer preserved from bootstrapping stages their `src` attributes
use a different fetchurl resulting in a changed hash.

pkgs/development/libraries/libbfd/default.nix

···
-{ stdenv
-, fetchurl, fetchpatch, gnu-config, autoreconfHook264, bison, binutils-raw
+{ stdenv, buildPackages
+, fetchurl, fetchpatch, gnu-config, autoreconfHook264, bison
 , libiberty, zlib
 }:
+
+let inherit (buildPackages.buildPackages) binutils-raw; in
 
 stdenv.mkDerivation rec {
   name = "libbfd-${version}";

pkgs/development/libraries/libiberty/default.nix

···
-{ stdenv, fetchurl, gcc, staticBuild ? false }:
+{ stdenv, buildPackages, fetchurl, staticBuild ? false }:
+
+let inherit (buildPackages.buildPackages) gcc; in
 
 stdenv.mkDerivation rec {
   name = "libiberty-${gcc.cc.version}";