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

This commit adds 'Ssstateen' extension, which is a supervisor-visible view
of the 'Smstateen' extension.  It means, this extension implements sstateen*
and hstateen* CSRs of the 'Smstateen' extension.

Note that 'Smstateen' extension itself is unchanged but due to
implementation simplicity, it is implemented so that 'Smstateen' implies
'Ssstateen' (just like 'M' implies 'Zmmul').

This is based on the latest version of RISC-V Profiles
(version 0.9-draft, Frozen):
<https://github.com/riscv/riscv-profiles/commit/226b7f643067b29abc6723fac60d5f6d3f9eb901>

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets): Update implication rules.
	(riscv_supported_std_s_ext) Add 'Ssstateen' extension.

gas/ChangeLog:

	* config/tc-riscv.c (enum riscv_csr_class): Rename
	CSR_CLASS_SMSTATEEN_AND_H{,_32} to CSR_CLASS_SSSTATEEN_...
	Add CSR_CLASS_SSSTATEEN.
	(riscv_csr_address): Support new/renamed CSR classes.
	* testsuite/gas/riscv/csr.s: Add 'Ssstateen' extension to comment.
	* testsuite/gas/riscv/csr-version-1p9p1.l: Reflect changes to
	error messages.
	* testsuite/gas/riscv/csr-version-1p10.l: Likewise.
	* testsuite/gas/riscv/csr-version-1p11.l: Likewise.
	* testsuite/gas/riscv/csr-version-1p12.l: Likewise.
	* testsuite/gas/riscv/ssstateen-csr.s: Test for 'Ssstateen' CSRs.
	* testsuite/gas/riscv/ssstateen-csr.d: Likewise.
	* testsuite/gas/riscv/smstateen-csr-s.d: Test to make sure that
	supervisor/hypervisor part of 'Smstateen' CSRs are accessible from
	'RV32IH_Smstateen', not just from 'RV32IH_Ssstateen' that is tested
	in ssstateen-csr.d.

include/ChangeLog:

	* opcode/riscv-opc.h: Update DECLARE_CSR declarations with
	new CSR classes.