RISC-V: Add `OP_V' to .insn named opcodes

RISC-V: Add `OP_V' to .insn named opcodes

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'.
	* testsuite/gas/riscv/insn.s: Add testcase.
	* testsuite/gas/riscv/insn.d: Likewise.
	* testsuite/gas/riscv/insn-dwarf.d: Reflect insn.s update.