Build dynamic iOS SDK.

- Places most ObjC code into webrtc/sdk/objc instead.
- New gyp targets to build, strip and export symbols for dylib.
- Removes old script used to generate dylib.

BUG=

Review URL: https://codereview.webrtc.org/1903663002

Cr-Commit-Position: refs/heads/master@{#12524}

webrtc/sdk/objc/Framework/Modules/module.modulemap

@@ -0,0 +1,6 @@
+framework module WebRTC {
+  umbrella header "WebRTC.h"
+
+  export *
+  module * { export * }
+}