dlfcn: Move dlsym into libc

The symbol was moved using scripts/move-symbol-to-libc.py.

In elf/Makefile, remove the $(libdl) dependency from testobj1.so
because it the unused libdl DSO now causes elf/tst-unused-deps to
fail.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>