RISC-V: Support S[sm]csrind extension csrs.

This patch supports RISC-V Smcsrind/Sscsrind privilege extension csrs.
Reuse csr 'smselect/siselect', 'mireg/sireg' and 'vsiselect,vsireg' csrs
in Smaia/Ssaia extension.

bfd/ChangeLog:

	* elfxx-riscv.c: New extensions.

gas/ChangeLog:

	* NEWS: Updated.
	* config/tc-riscv.c (enum riscv_csr_class): New extensions.
	(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 extensions.

include/ChangeLog:

	* opcode/riscv-opc.h (CSR_MIREG2): New csr.
	(CSR_MIREG3): Ditto.
	(CSR_MIREG4): Ditto.
	(CSR_MIREG5): Ditto.
	(CSR_MIREG6): Ditto.
	(CSR_SIREG2): Ditto.
	(CSR_SIREG3): Ditto.
	(CSR_SIREG4): Ditto.
	(CSR_SIREG5): Ditto.
	(CSR_SIREG6): Ditto.
	(CSR_VSIREG2): Ditto.
	(CSR_VSIREG3): Ditto.
	(CSR_VSIREG4): Ditto.
	(CSR_VSIREG5): Ditto.
	(CSR_VSIREG6): Ditto.
	(DECLARE_CSR): Ditto.