asan: _bfd_elf_parse_attributes heap buffer overflow

I exposed a problem with the change in commit 574ec108 to the outer
loop of _bfd_elf_parse_attributes.  "p_end - p >= 4" is better than
"p < p_end - 4" as far as pointer UB is concerned if the size of the
attritbute section is say, 3 bytes.  However you do need to ensure p
never exceeds p_end, and that length remaining is kept consistent with
the pointer.

	* elf-attrs.c (elf_attr_strdup): New function.
	(_bfd_elf_attr_strdup): Use it here.
	(elf_add_obj_attr_string): New function, extracted from..
	(bfd_elf_add_obj_attr_string): ..here.
	(elf_add_obj_attr_int_string): New function, extracted from..
	(bfd_elf_add_obj_attr_int_string): ..here.
	(_bfd_elf_parse_attributes): Don't allocate an extra byte for a
	string terminator.  Instead ensure parsing doesn't go past
	end of sub-section.  Use size_t variables for lengths.