re PR c++/15428 (vtables shouldn't always be weak on Darwin)

	PR c++/15428
	* default.h (TARGET_WEAK_NOT_IN_ARCHIVE_TOC): New name
	for TARGET_EXPLICIT_INSTANTIATIONS_ONE_ONLY, with reversed sense.
	* config/darwin.h (TARGET_WEAK_NOT_IN_ARCHIVE_TOC): Likewise.
	* doc/tm.texi (TARGET_WEAK_NOT_IN_ARCHIVE_TOC): Rewrite
	documentation to reflect the new macro name and to clarify its
	meaning.
	* cp/decl2.c (maybe_emit_vtables): If TARGET_WEAK_NOT_IN_ARCHIVE_TOC
	is nonzero, and if we see a noninline definition of a key method,
	make the vtables nonweak.

From-SVN: r82610