Export ___gxx_personality_v0 as a weak symbol on iOS

This is apparently required by the latest Clang toolchain in order to do a
bitcode build of the engine with LTO enabled.

See https://github.com/flutter/flutter/issues/110140

Change-Id: I6eec2e741e5a9a67aece176aab365303b5425827
Reviewed-on: https://flutter-review.googlesource.com/c/third_party/libcxxabi/+/33360
Reviewed-by: Zach Anderson <zra@google.com>