gnu-efi: Use their crosscompilation support correctly

IIUC, previously, the cross-compilation support was done in a somewhat
hacky way and was, basically, special-cased for ARM.

Now we use the cross-compilation support intergrated into their own
build system.

Test:

* nix-build --arg crossSystem '(import <nixpkgs/lib>).systems.examples.musl64' '<nixpkgs>' -A gnu-efi

pkgs/development/libraries/gnu-efi/default.nix

@@ -1,4 +1,6 @@
-{ stdenv, fetchurl, pciutils }: with stdenv.lib;
+{ stdenv, buildPackages, fetchurl, pciutils }:
+
+with stdenv.lib;
 
 stdenv.mkDerivation rec {
   pname = "gnu-efi";
@@ -15,14 +17,9 @@ stdenv.mkDerivation rec {
 
   makeFlags = [
     "PREFIX=\${out}"
-    "CC=${stdenv.cc.targetPrefix}gcc"
-    "AS=${stdenv.cc.targetPrefix}as"
-    "LD=${stdenv.cc.targetPrefix}ld"
-    "AR=${stdenv.cc.targetPrefix}ar"
-    "RANLIB=${stdenv.cc.targetPrefix}ranlib"
-    "OBJCOPY=${stdenv.cc.targetPrefix}objcopy"
-  ] ++ stdenv.lib.optional stdenv.isAarch32 "ARCH=arm"
-    ++ stdenv.lib.optional stdenv.isAarch64 "ARCH=aarch64";
+    "HOSTCC=${buildPackages.stdenv.cc.targetPrefix}cc"
+    "CROSS_COMPILE=${stdenv.cc.targetPrefix}"
+  ];
 
   meta = with stdenv.lib; {
     description = "GNU EFI development toolchain";