Add symlink to src/third_party/libc++-static

This is used on Mac to link against a precompiled libc++
library on Mac (see https://crbug.com/400091 for details)

TBR=tkchin@webrtc.org
TESTED=Verified the warnings like:
ld: warning: directory not found for option '-L../../third_party/libc++-static'
are no longer printed on Mac.

Review URL: https://codereview.webrtc.org/1475643010 .

Cr-Commit-Position: refs/heads/master@{#10797}

setup_links.py

@@ -42,6 +42,7 @@ DIRECTORIES = [
   'third_party/expat',
   'third_party/instrumented_libraries',
   'third_party/jsoncpp',
+  'third_party/libc++-static',
   'third_party/libjpeg',
   'third_party/libjpeg_turbo',
   'third_party/libsrtp',