Prepare for restoration of .interp section in libpthread.so

Make runtime-linker.h available outside $(elf-objpfx) by moving
the file to $(common-objpfx) and the rules for it to Makerules.

Tested for x86_64 and x86 (testsuite, and that no compiled code
changed by the patch).

* Makeconfig (+interp): Remove unused variable.
* elf/Makefile ($(objpfx)interp.os): Define for [$(build-shared) = yes]
only.  Depend on $(common-objpfx)runtime-linker.h instead of
$(elf-objpfx)runtime-linker.h.
($(elf-objpfx)runtime-linker.h): Rename to
$(common-objpfx)runtime-linker.h and move ...
* Makerules [$(build-shared) = yes]: ... here.
* elf/interp.c: Include <runtime-linker.h> instead of
<elf/runtime-linker.h>.