RISC-V: Add `OP_V' to .insn directive

This commit adds `OP_V' (OP-V: vector instruction opcode for now
ratified `V' extension) to .insn opcode name list.  Although vector
instruction encoding is not implemented in `.insn' directive, it will
help future implementation of custom vector `.insn'.

gas/ChangeLog:

	* config/tc-riscv.c (opcode_name_list): Add `OP_V'.

gas/config/tc-riscv.c

@@ -749,7 +749,7 @@ static const struct opcode_name_t opcode_name_list[] =
   {"NMADD",     0x4f},
   {"NMSUB",     0x4b},
   {"OP_FP",     0x53},
-  /*reserved    0x57.  */
+  {"OP_V",      0x57},
   {"CUSTOM_2",  0x5b},
   /* 48b        0x5f.  */