dlfcn: Cleanups after -ldl is no longer required

This commit removes the ELF constructor and internal variables from
dlfcn/dlfcn.c.  The file now serves the same purpose as
nptl/libpthread-compat.c, so it is renamed to dlfcn/libdl-compat.c.
The use of libdl-shared-only-routines ensures that libdl.a is empty.

This commit adjusts the test suite not to use $(libdl).  The libdl.so
symbolic link is no longer installed.

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