RISC-V: Fix bug in prior addi/c.nop patch.
    
    	gas/
    	* config/tc-riscv.c (validate_riscv_insn) <'z'>: New.
    	(riscv_ip) <'z'>: New.
    	opcodes/
    	* riscv-opc.c (riscv_opcodes) <addi>: Use z instead of 0.