RISC-V: Fix required bits on certain environments

If `unsigned long long' type has more than 64-bits, validate_riscv_insn
function generated wrong required_bits value.  This commit fixes this
small issue (may be too pedantic though).

gas/ChangeLog:

	* config/tc-riscv.c (validate_riscv_insn): Compute correct
	required_bits value when unsigned long long is larger than 64b.

gas/config/tc-riscv.c

@@ -1110,7 +1110,7 @@ validate_riscv_insn (const struct riscv_opcode *opc, int length)
   else
     insn_width = 8 * length;
 
-  required_bits = ~0ULL >> (64 - insn_width);
+  required_bits = ((unsigned long long)(insn_t)~0ULL) >> (64 - insn_width);
 
   if ((used_bits & opc->match) != (opc->match & required_bits))
     {