Add back link_settings for iOS.

In https://codereview.chromium.org/111723007, link_settings was dropped on
all platforms including iOS. iOS does need link_settings (see
post-checkin comments in the CL aforementioned).

BUG=72633
TEST=Once this is rolled in, ios* bots pass *unittests without an error in loading icudtl.dat.
R=mark@chromium.org

Review URL: https://codereview.chromium.org/118373006

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu46@241739 4ff67af0-8c30-449e-8e8b-ad334ec8d88c