Add slack-desktop for aarch64 on a native Electron runtime

Slack only publishes an x86_64 Linux build, so the AUR recipe produces an
aarch64 package full of x86_64 binaries that cannot start. Run Slack's
application code on the AArch64 build of the Electron release it ships
with, replace the three native addons Slack requires at boot with
JavaScript equivalents, and drop the x86_64 addons it can do without.

pkgbuilds/slack-desktop/electron-native-auth.js

@@ -0,0 +1,14 @@
+// Linux builds of electron-native-auth (github.com/clavin/electron-native-auth)
+// compile src/addon_none.cc: native auth sessions are unavailable and
+// constructing a request throws. This is the same facade in JavaScript.
+class AuthRequest {
+  static isAvailable() {
+    return false;
+  }
+
+  constructor() {
+    throw new Error('this function is not implemented for this platform');
+  }
+}
+
+exports.AuthRequest = AuthRequest;