Polish the treatment of dl-tunable-list.h in Makeconfig.

Like basically all before-compile headers, dl-tunable-list.h should be
generated using a stamp file to minimize unnecessary rebuilding; it
wasn't being added to common-generated, so it wouldn't get cleaned up;
and it was in between the rules for libc-modules.h/.stmp and their own
addition to common-generated.

	* Makeconfig (dl-tunable-list.h): Generate using a stamp file.
	(common-generated): Add libc-modules.h and libc-modules.stmp in a
	more appropriate location. Also add dl-tunable-list.h and
	dl-tunable-list.stmp.