Set ZERO_AR_DATE=1 when running libtool.

Ref: http://www.opensource.apple.com/source/cctools/cctools-809/misc/libtool.c

Adhoc testing with base_unittests reduced non-deterministic bytes from ~2347 to
~174. It's definitely the lowest hanging fruit, which will permit us to focus on
the remaining bytes.

R=thakis@chromium.org
BUG=chromium:330262

Review URL: https://codereview.chromium.org/699083004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1998 78cadc50-ecff-11dd-a971-7dbc132099af