ENABLE_CHECKING in bfd, opcodes, binutils, ld

gas already has this.  Here it enables checking hash table type passed
to elf_link_hash_lookup and elf_link_hash_traverse.

bfd/
	* elf-bfd.h (ENABLE_CHECKING): Define.
	(elf_link_hash_lookup): Abort if wrong type of hash table.
	(elf_link_hash_traverse): Likewise.
	* configure.ac (--enable-checking): Add support.
	* config.in: Regenerate.
	* configure: Regenerate.
binutils/
	* configure.ac (--enable-checking): Add support.
	* config.in: Regenerate.
	* configure: Regenerate.
ld/
	* configure.ac (--enable-checking): Add support.
	* config.in: Regenerate.
	* configure: Regenerate.
opcodes/
	* configure.ac (--enable-checking): Add support.
	* config.in: Regenerate.
	* configure: Regenerate.