Enable 64-bit build of host toolset

When building v8 mksnapshot host toolset as 64-bit, it requires icu to
be 64-bit also.

BUG=346626
R=jochen@chromium.org, jshin@chromium.org

Review URL: https://codereview.chromium.org/212593002

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu46@262949 4ff67af0-8c30-449e-8e8b-ad334ec8d88c