aarch64: Expand maximum number of operands from 5 to 6

Given the introduction of the new Armv9.4-a `sysp' insn using the
following syntax:

	sysp #<op1>, <Cn>, <Cm>, #<op2>{, <Xt1>, <Xt2>}

and by extension the need to encode 6 assembly operands, extend
Binutils to handle instructions taking 6 operands, up from a previous
maximum of 5.