Include .interp section only for libc.so

Barring libc.so and libdl.so, none of the libraries have any entry
points, so it is pointless to add a .interp section for them.  The
libdl.so entry point (in dlfcn/eval.c) is also defunct, so remove that
file as well.

Build tested for x86_64, ppc64 and s390x.  I have not moved
CFLAGS-interp.c to CPPFLAGS-interp.c isnce I'll be removing it
completely in a follow-up patch.

Siddhesh

	* Makerules (lib%.so): Don't include $(+interp) in
	prerequisites.
	* elf/Makefile (CFLAGS-interp.c): Don't define NOT_IN_libc.
	* dlfcn/eval.c: Remove file.