RISC-V: Add Smrnmi extension csrs.

This patch support Smrnmi extension[1],
The csrs address can be find in[2].

[1] https://github.com/riscv/riscv-isa-manual/commit/35eb3948bf0b87c83fab5a7238bd68b6211faf62
[2] https://github.com/riscv/riscv-isa-manual/blob/smrnmi-1.0/src/priv-csrs.adoc

bfd/ChangeLog:

	* elfxx-riscv.c: New extension.

gas/ChangeLog:

	* NEWS: Add Smrnmi extension support.
	* config/tc-riscv.c (enum riscv_csr_class): New extension class.
	(riscv_csr_address): Ditto.
	* testsuite/gas/riscv/csr-version-1p10.d: New csrs.
	* testsuite/gas/riscv/csr-version-1p10.l: Ditto.
	* testsuite/gas/riscv/csr-version-1p11.d: Ditto.
	* testsuite/gas/riscv/csr-version-1p11.l: Ditto.
	* testsuite/gas/riscv/csr-version-1p12.d: Ditto.
	* testsuite/gas/riscv/csr-version-1p12.l: Ditto.
	* testsuite/gas/riscv/csr.s:  Ditto.
	* testsuite/gas/riscv/march-help.l: New extension.

include/ChangeLog:

	* opcode/riscv-opc.h (CSR_MNSCRATCH): New csr.
	(CSR_MNEPC): Ditto.
	(CSR_MNCAUSE): Ditto.
	(CSR_MNSTATUS): Ditto.
	(DECLARE_CSR): New csr declarations.