RISC-V: Support the read-only CSR checking.

CSRRW and CSRRWI always write CSR.  CSRRS, CSRRC, CSRRSI and CSRRCI write CSR
when RS1 isn't zero.  The CSR is read only if the [11:10] bits of CSR address
is 0x3.  The read-only CSR can not be written by the CSR instructions.

	gas/
	* config/tc-riscv.c (riscv_ip): New boolean insn_with_csr to indicate
	we are assembling instruction with CSR.  Call riscv_csr_read_only_check
	after parsing all arguments.
	(enum csr_insn_type): New enum is used to classify the CSR instruction.
	(riscv_csr_insn_type, riscv_csr_read_only_check): New functions.  These
	are used to check if we write a read-only CSR by the CSR instruction.

	* testsuite/gas/riscv/priv-reg-fail-read-only-01.s: New testcase.  Test
	all CSR for the read-only CSR checking.
	* testsuite/gas/riscv/priv-reg-fail-read-only-01.d: Likewise.
	* testsuite/gas/riscv/priv-reg-fail-read-only-01.l: Likewise.
	* testsuite/gas/riscv/priv-reg-fail-read-only-02.s: New testcase.  Test
	all CSR instructions for the read-only CSR checking.
	* testsuite/gas/riscv/priv-reg-fail-read-only-02.d: Likewise.
	* testsuite/gas/riscv/priv-reg-fail-read-only-02.l: Likewise.