RISC-V: Add 'Sstc' extension and its CSRs

RISC-V: Add 'Sstc' extension and its CSRs

This commit adds "stimecmp / vstimecmp" Extension (Sstc) and its CSRs.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_supported_std_s_ext): Add 'Sstc'
	extension to valid 'S' extension list.

gas/ChangeLog:

	* config/tc-riscv.c (enum riscv_csr_class): Add CSR classes for
	'Sstc' extension. (riscv_csr_address): Add handling for new CSR
	classes.
	* testsuite/gas/riscv/csr-dw-regnums.s: Add new CSRs.
	* testsuite/gas/riscv/csr-dw-regnums.d: Likewise.
	* testsuite/gas/riscv/csr.s: Add new CSRs.
	* testsuite/gas/riscv/csr-version-1p9p1.d: Likewise.
	* testsuite/gas/riscv/csr-version-1p9p1.l: Likewise.
	* testsuite/gas/riscv/csr-version-1p10.d: Likewise.
	* testsuite/gas/riscv/csr-version-1p10.l: Likewise.
	* testsuite/gas/riscv/csr-version-1p11.d: Likewise.
	* testsuite/gas/riscv/csr-version-1p11.l: Likewise.
	* testsuite/gas/riscv/csr-version-1p12.d: Likewise.
	* testsuite/gas/riscv/csr-version-1p12.l: Likewise.

include/ChangeLog:

	* opcode/riscv-opc.h (CSR_STIMECMP, CSR_STIMECMPH,
	CSR_VSTIMECMP, CSR_VSTIMECMPH): New CSR macros.