Power10 bignum operands

When built on a 32-bit host without --enable-64-bit-bfd, powerpc-linux
and other 32-bit powerpc targeted binutils fail to assemble some
power10 prefixed instructions with 34-bit fields.  A typical error
seen when running the testsuite is
.../gas/testsuite/gas/ppc/prefix-pcrel.s:10: Error: bignum invalid
In practice this doesn't matter for addresses:  32-bit programs don't
need or use the top 2 bits of a d34 field when calculating addresses.
However it may matter when loading or adding 64-bit constants with
paddi.  A power10 processor in 32-bit mode still has 64-bit wide GPRs.
So this patch enables limited support for O_big PowerPC operands, and
corrects sign extension of 32-bit constants using X_extrabit.

	* config/tc-ppc.c (insn_validate): Use uint64_t for operand values.
	(md_assemble): Likewise.  Handle bignum operands.
	(ppc_elf_suffix): Handle O_big.  Remove unnecessary input_line_pointer
	check.
	* expr.c: Delete unnecessary forward declarations.
	(generic_bignum_to_int32): Return uint32_t.
	(generic_bignum_to_int64): Return uint64_t.  Compile always.
	(operand): Twiddle X_extrabit for unary '~'.  Set X_unsigned and
	clear X_extrabit for unary '!'.
	* expr.h (generic_bignum_to_int32): Declare.
	(generic_bignum_to_int64): Declare.
	* testsuite/gas/ppc/prefix-pcrel.s,
	* testsuite/gas/ppc/prefix-pcrel.d: Add more instructions.