Revert "stdenv: aarch64 is also ARM"

This reverts commit a5f4e22289229a5f6b7341d17c45db98be84d479.

Breaks the stdenv build, e.g.: http://hydra.nixos.org/build/50015717
In general, the architectures are different enough that there is no
reason to consider both as ARM, just like we don't consider x86_64 as
32-bit x86.

cc @fpletz

pkgs/development/libraries/libdrm/default.nix

···
     "echo : \\\${ac_cv_func_clock_gettime=\'yes\'} > config.cache";
 
   configureFlags = [ "--disable-valgrind" ]
-    ++ stdenv.lib.optionals stdenv.isArm [ "--enable-tegra-experimental-api" "--enable-etnaviv-experimental-api" ]
+    ++ stdenv.lib.optionals (stdenv.isArm || stdenv.isAarch64) [ "--enable-tegra-experimental-api" "--enable-etnaviv-experimental-api" ]
     ++ stdenv.lib.optional stdenv.isDarwin "-C";
 
   crossAttrs.configureFlags = configureFlags ++ [ "--disable-intel" ];

pkgs/development/libraries/mesa/default.nix

···
 
 let
   defaultGalliumDrivers =
-    if stdenv.isArm
+    if (stdenv.isArm || stdenv.isAarch64)
     then ["nouveau" "freedreno" "vc4" "etnaviv"]
     else ["i915" "ilo" "r300" "r600" "radeonsi" "nouveau"];
   defaultDriDrivers =
-    if stdenv.isArm
+    if (stdenv.isArm || stdenv.isAarch64)
     then ["nouveau"]
     else ["i915" "i965" "nouveau" "radeon" "r200"];
   defaultVulkanDrivers =
-    if stdenv.isArm
+    if (stdenv.isArm || stdenv.isAarch64)
     then []
     else ["intel"] ++ lib.optional enableRadv "radeon";
 in

pkgs/stdenv/generic/default.nix

···
             || system == "mips64el-linux";
       isArm = system == "armv5tel-linux"
            || system == "armv6l-linux"
-           || system == "armv7l-linux"
-           || system == "aarch64-linux";
+           || system == "armv7l-linux";
       isAarch64 = system == "aarch64-linux";
       isBigEndian = system == "powerpc-linux";