Move UNDEFWEAK_NO_DYNAMIC_RELOC to elf-bfd.h

Move UNDEFWEAK_NO_DYNAMIC_RELOC to elf-bfd.h so that it can be used by
other ELF linker backends.

	* elf32-ppc.c (UNDEFWEAK_NO_DYNAMIC_RELOC): Moved to ...
	* elf-bfd.h (UNDEFWEAK_NO_DYNAMIC_RELOC): Here.
	* elf64-ppc.c (UNDEFWEAK_NO_DYNAMIC_RELOC): Removed.