Allow both signed and unsigned fields in PowerPC cmpli insn

There are legitimate reasons to allow a signed value in a cmpli insn
field, for example to test for a "stw r1,lock@sdarel(r13)" instruction
in user code, a kernel might use
	subis r3,r3,STW_R1_0R13@ha	# subtract off high part
	cmplwi r3,lock@sdarel		# is low part accessing lock?
Since the lock@sdarel may take a range of -32768 to 32767,
the allowed range of cmpli immediate must be at least [-32768,65535].

bfd/
	* elf32-ppc.c (ppc_elf_relocate_section): Treat field of cmpli
	insn as a bitfield; Use complain_overflow_bitfield.
	* elf64-ppc.c (ppc64_elf_relocate_section): Likewise.
opcodes/
	* ppc-opc.c (UISIGNOPT): Define and use with cmpli.
gas/
	* config/tc-ppc.c (ppc_insert_operand): Handle PPC_OPERAND_SIGNOPT
	on unsigned fields.  Comment on PPC_OPERAND_SIGNOPT signed fields
	in 64-bit mode.
gold/
	* powerpc.cc (relocate): Treat field of cmpli insn as a bitfield.
PROVIDE in linker script vs. built-in symbols

	* ldexp.c (exp_fold_tree_1 <etree_provide>): Make PROVIDEd
	linker script symbol value override a built-in linker symbol.