RISC-V: avoid use of match_opcode() in riscv_insn_types[]

RISC-V: avoid use of match_opcode() in riscv_insn_types[]

As of 27b33966b18e ("RISC-V: disallow x0 with certain macro-insns") the
.match_func field may be NULL for entries used for assembly only, which
is the case for the entire table. With .match and .mask both zero the
function would only ever succeed anyway. Save almost a hundred base
relocations in the final executable by using NULL instead.