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: I1dea9137c79c5f26b493640cfae0856c1a7ea301