Link libgtest_main before libgtest

libgtest_main uses symbols from libgtest.  If libgtest_main is linked
after libgtest, and the test .o files don't reference libgtest (which is
unlikely), then the libgtest .o files will be ignored and libgtest_main
won't be able to find its symbols from libgtest.

Change-Id: Ica3bc679bd9d14e9a3065be927f085cf52faab64