Introduce a separate linker config section for native tests.

There are tests that require access to both system and vendor libraries,
like what the linker sets up when there is no matching config section.

Test: atest cameraservice_test libsurfaceflinger_unittest perfprofd_test \
  inputflinger_tests (no new failures, but 3 old ones)
Test: Internal test can load libandroid.so
Bug: 124127405
Bug: 123700170

Change-Id: I4f5cb2e09bdf1ae510259198c08c252b41249d94