multiple definition warnings from script symbols

In commit 89753bbf I enabled a warning for scripts redefining
symbols.  The idea was to not warn for symbols defined by shared
libraries (the h->u.def.section->output_section != NULL test), but the
test failed to take into account absolute symbols.  Absolute symbols
defined in shared libraries are currently indistinguishable from
absolute symbols defined in relocatable objects, at least when only
looking at struct bfd_link_hash_entry.  So, don't warn for any
absolute symbols.

	* ldexp.c (update_definedness): Don't return false for absolute
	symbols.
	* ldmain.c (multiple_definition): Print "warning: " in message
	when not a hard error.