Don't output symbol version definitions for non-DT_NEEDED libs

	PR 16452, 16457
	* elflink.c (_bfd_elf_link_find_version_dependencies): Exclude
	symbols from libraries that won't be listed in DT_NEEDED.
	(elf_link_output_extsym): Don't output verdefs for such symbols.
Don't deprecate powerpc mftb insn

mftb is marked phased out in the architecture manual, but we can keep
it as an extended mnemonic for mftbl.

	* ppc-opc.c (powerpc_opcodes <mftb>): Don't deprecate for power7.
	(TB): Delete.
	(insert_tbr, extract_tbr): Validate tbr number.
Power4 should treat mftb as extended mfspr mnemonic

On further reading of ISA manual it appears gas should have been
treating mftb and mftbu as extended mnemonics for mfspr, for ISA 2.03
and later.

opcodes/
	* ppc-opc.c (powerpc_opcodes): Make mftb* generate mfspr for
	power4 and later.
gas/testsuite/
	* gas/ppc/a2.d: Update for mftb change.
	* gas/ppc/476.d: Likewise.
Automatic date update in version.in