treewide: Get rid of *Platform.arch

Use `parsed.cpu.name` or `platform.gcc.arch` instead.

pkgs/development/libraries/fontconfig/2.10.nix

@@ -24,7 +24,9 @@ stdenv.mkDerivation rec {
   ];
 
   # We should find a better way to access the arch reliably.
-  crossArch = hostPlatform.arch or null;
+  crossArch = if stdenv.hostPlatform != stdenv.buildPlatform
+    then hostPlatform.parsed.cpu.name
+    else null;
 
   preConfigure = ''
     if test -n "$crossConfig"; then

pkgs/development/libraries/fontconfig/default.nix

@@ -56,7 +56,9 @@ stdenv.mkDerivation rec {
   ];
 
   # We should find a better way to access the arch reliably.
-  crossArch = hostPlatform.arch or null;
+  crossArch = if stdenv.hostPlatform != stdenv.buildPlatform
+    then hostPlatform.parsed.cpu.name
+    else null;
 
   preConfigure = ''
     if test -n "$crossConfig"; then