RISC-V: Fallback for instructions longer than 64b

We don't support instructions longer than 64-bits yet.  Still, we can
modify validate_riscv_insn function to prevent unexpected behavior by
limiting the "length" of an instruction to 64-bit (or less).

gas/ChangeLog:

	* config/tc-riscv.c (validate_riscv_insn): Fix function
	description comment based on current spec.  Limit instruction
	length up to 64-bit for now.  Make sure that required_bits does
	not corrupt even if unsigned long long is longer than 64-bit.