qtwebengine: implement meta.platforms correctly

pkgs/development/libraries/qt-5/modules/qtwebengine.nix

@@ -240,7 +240,22 @@ qtModule {
   meta = with lib; {
     description = "A web engine based on the Chromium web browser";
     maintainers = with maintainers; [ matthewbauer ];
-    platforms = platforms.unix;
+
+    # qtwebengine-5.15.8: "QtWebEngine can only be built for x86,
+    # x86-64, ARM, Aarch64, and MIPSel architectures."
+    platforms =
+      lib.trivial.pipe lib.systems.doubles.all [
+        (map (double: lib.systems.elaborate { system = double; }))
+        (lib.lists.filter (parsedPlatform: with parsedPlatform;
+          isUnix &&
+          (isx86_32  ||
+           isx86_64  ||
+           isAarch32 ||
+           isAarch64 ||
+           (isMips && isLittleEndian))))
+        (map (plat: plat.system))
+      ];
+
     # This build takes a long time; particularly on slow architectures
     timeout = 24 * 3600;
     # we are still stuck with MacOS SDK 10.12 on x86_64-darwin