Move libdlext_... to NATIVE_TESTS instead of ETC

ETC did end up causing problems, since it's not per-arch. But
SHARED_LIBRARIES isn't correct either, since the build system tries to
read shared libraries as ELF files (to create the toc files).

NATIVE_TESTS is slightly better -- they are not libraries, so we don't
attempt to generate toc files, but they are considered per-arch.

Bug: 38463793
Test: Run CtsBionicTestCases
Change-Id: I01b4b093359e0c39eb2f8639d5c5dde9e304ed20