Fix fat build for ios.
Fix fat build for ios.

root_build_dir will always be the root build output directory.
root_out_dir is the directory "for the current toolchain".

WebRTC.framework is always in the root output directory.

Bug: webrtc:7507
Change-Id: I30b8eccaac3ed07e40c86acf361ee24a1c20b074
Reviewed-on: https://webrtc-review.googlesource.com/7640
Commit-Queue: Kári Helgason <79949f6d84ac41ff1f64c59fd5a554be5ff12a02@webrtc.org>
Reviewed-by: Kári Helgason <79949f6d84ac41ff1f64c59fd5a554be5ff12a02@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20237}
ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc