Do not set entry-point for Windows

Bug: http://b/62424007

This breaks 64-bit Windows executables.  Probably due to the Clang
migration, the entry-point need not be set even for 32-bit executables.

Test: 32-bit and 64-bit aapt2_tests.exe and ziparchive-tests.exe can
start.  (There are some failed tests, though).

Change-Id: Id3a7d3d2802f6b671332afff4558840bdabddeb5