gcc: fix building darwin->linux cross compiler

This reverts d9feea5 with some slight modifications to work with
other changes since then.

Fixes #88213.

pkgs/development/compilers/gcc/7/default.nix

@@ -9,7 +9,7 @@
 , enableLTO ? true
 , texinfo ? null
 , perl ? null # optional, for texi2pod (then pod2man)
-, gmp, mpfr, libmpc, gettext, which
+, gmp, mpfr, libmpc, gettext, which, patchelf
 , libelf                      # optional, for link-time optimizations (LTO)
 , isl ? null # optional, for the Graphite optimization framework.
 , zlib ? null
@@ -155,11 +155,14 @@ stdenv.mkDerivation ({
 
   # For building runtime libs
   depsBuildTarget =
-    if hostPlatform == buildPlatform then [
-      targetPackages.stdenv.cc.bintools # newly-built gcc will be used
-    ] else assert targetPlatform == hostPlatform; [ # build != host == target
-      stdenv.cc
-    ];
+    (
+      if hostPlatform == buildPlatform then [
+        targetPackages.stdenv.cc.bintools # newly-built gcc will be used
+      ] else assert targetPlatform == hostPlatform; [ # build != host == target
+        stdenv.cc
+      ]
+    )
+    ++ optional targetPlatform.isLinux patchelf;
 
   buildInputs = [
     gmp mpfr libmpc libelf