Make libdexfile usable from libunwind

Can not depend on libutils as that causes build cycle.
Use just jni_headers instead.

Make the library available in vendor builds.

Fill one of the basic dex Open methods which does not
depend on the ART runtime.

Add some needed definitions to elf.h for libunwind.

Bug: 66919073
Test: m test-art-host-gtest
Change-Id: Ie3a42c127a19c1b986934723688e9f8903ba8fb9