stdenv: Rename `isGNU` to `isHurd` as GNU is a userland

Elsewhere, things called GNU indeed includes GNU/Linux or GNU/Hurd, but this
predicate was defined excluding Linux regardless of userland.

lib/systems/inspect.nix

···
     Linux        = { kernel = kernels.linux; };
     SunOS        = { kernel = kernels.solaris; };
     FreeBSD      = { kernel = kernels.freebsd; };
+    Hurd         = { kernel = kernels.hurd; };
     NetBSD       = { kernel = kernels.netbsd; };
     OpenBSD      = { kernel = kernels.openbsd; };
     Windows      = { kernel = kernels.windows; };
···
 
     Arm32        = recursiveUpdate patterns.Arm patterns."32bit";
     Arm64        = recursiveUpdate patterns.Arm patterns."64bit";
-
   };
 
   predicates = mapAttrs'

lib/systems/parse.nix

···
   {
     darwin  = { execFormat = macho;   families = { inherit unix; }; };
     freebsd = { execFormat = elf;     families = { inherit unix bsd; }; };
+    hurd    = { execFormat = elf;     families = { inherit unix; }; };
     linux   = { execFormat = elf;     families = { inherit unix; }; };
     netbsd  = { execFormat = elf;     families = { inherit unix bsd; }; };
     none    = { execFormat = unknown; families = { inherit unix; }; };
···
   mkSkeletonFromList = l: {
     "2" = # We only do 2-part hacks for things Nix already supports
       if elemAt l 1 == "cygwin"
-        then { cpu = elemAt l 0;                      kernel = "windows"; abi = "cygnus";    }
+        then { cpu = elemAt l 0;                      kernel = "windows";  abi = "cygnus";   }
+      else if elemAt l 1 == "gnu"
+        then { cpu = elemAt l 0;                      kernel = "hurd";     abi = "gnu";      }
       else   { cpu = elemAt l 0;                      kernel = elemAt l 1;                   };
     "3" = # Awkwards hacks, beware!
       if elemAt l 1 == "apple"

pkgs/development/compilers/gcc/4.8/default.nix

···
   libc_dev = stdenv.cc.libc_dev;
 
   postPatch =
-    if (stdenv.isGNU
+    if (stdenv.isHurd
         || (libcCross != null                  # e.g., building `gcc.crossDrv'
             && libcCross ? crossConfig
             && libcCross.crossConfig == "i586-pc-gnu")

pkgs/development/compilers/gcc/4.9/default.nix

···
   libc_dev = stdenv.cc.libc_dev;
 
   postPatch =
-    if (stdenv.isGNU
+    if (stdenv.isHurd
         || (libcCross != null                  # e.g., building `gcc.crossDrv'
             && libcCross ? crossConfig
             && libcCross.crossConfig == "i586-pc-gnu")

pkgs/development/compilers/gcc/5/default.nix

···
   '';
 
   postPatch =
-    if (stdenv.isGNU
+    if (stdenv.isHurd
         || (libcCross != null                  # e.g., building `gcc.crossDrv'
             && libcCross ? crossConfig
             && libcCross.crossConfig == "i586-pc-gnu")

pkgs/development/compilers/gcc/6/default.nix

···
   hardeningDisable = [ "format" ];
 
   postPatch =
-    if (stdenv.isGNU
+    if (stdenv.isHurd
         || (libcCross != null                  # e.g., building `gcc.crossDrv'
             && libcCross ? crossConfig
             && libcCross.crossConfig == "i586-pc-gnu")

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

···
   hardeningDisable = [ "format" ];
 
   postPatch =
-    if (stdenv.isGNU
+    if (stdenv.isHurd
         || (libcCross != null                  # e.g., building `gcc.crossDrv'
             && libcCross ? crossConfig
             && libcCross.crossConfig == "i586-pc-gnu")

pkgs/stdenv/generic/default.nix

···
 
       # Utility flags to test the type of platform.
       inherit (hostPlatform)
-        isDarwin isLinux isSunOS isCygwin isFreeBSD isOpenBSD isi686 isx86_64
-        is64bit isMips isBigEndian;
+        isDarwin isLinux isSunOS isHurd isCygwin isFreeBSD isOpenBSD
+        isi686 isx86_64 is64bit isMips isBigEndian;
       isArm = hostPlatform.isArm32;
       isAarch64 = hostPlatform.isArm64;
-      # Other code instead checks for anything using GNU userland,
-      # e.g. GNU/linux. This refers just to GNU Hurd.
-      isGNU = system == "i686-gnu";
 
       # Whether we should run paxctl to pax-mark binaries.
       needsPax = isLinux;