Set dynobj to a normal input file if possible

Set dynobj to a normal input file if possible

When check_relocs is called after gc-sections has run,
_bfd_elf_link_create_dynstrtab may be called with an dynamic object
and hash_table->dynobj may be NULL.  We may not set dynobj, an input
file holding linker created dynamic sections to the dynamic object,
which has its own dynamic sections.  We need to find a normal input
file to hold linker created sections if possible.  Otherwise ld will
crash during LTO input rescan when linker created dynamic section
overrides input dynamic section.

	* elflink.c (_bfd_elf_link_create_dynstrtab): Set dynobj to a
	normal input file if possible.