Add libdl.so to the list of system shared libs

Note that libdl is already (always) implicitly loaded
because libc.so has a dependency on it.

Also make sure that libc.so always preceding libdl.so
in dt_needed list.

Bug: http://b/62815515
Test: make
Change-Id: I94c9d676b7fa98438b452d24f6c3bbf93166c6a9