RISC-V: Support version controling for ISA standard extensions

 - New option -misa-spec support: -misa-spec=[2.2|20190608|20191213] and
   corresponding configuration option --with-isa-spec.

 - Current default ISA spec set to 2.2, but we intend to bump this to
   20191213 or later in next release.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c (riscv_ext_version): New.
	(riscv_ext_version_table): Ditto.
	(get_default_version): Ditto.
	(riscv_subset_t::implied_p): New field.
	(riscv_subset_t::riscv_subset_t): Init implied_p.
	(riscv_subset_list::add): New.
	(riscv_subset_list::handle_implied_ext): Pass riscv_subset_t
	instead of separated argument.
	(riscv_subset_list::to_string): Handle zifencei and zicsr, and
	omit version if version is unknown.
	(riscv_subset_list::parsing_subset_version): New argument `ext`,
	remove default_major_version and default_minor_version, get
	default version info via get_default_version.
	(riscv_subset_list::parse_std_ext): Update argument for
	parsing_subset_version calls.
	Handle 2.2 ISA spec, always enable zicsr and zifencei, they are
	included in baseline ISA in that time.
	(riscv_subset_list::parse_multiletter_ext): Update argument for
	`parsing_subset_version` and `add` calls.
	(riscv_subset_list::parse): Adjust argument for
	riscv_subset_list::handle_implied_ext call.
	* config.gcc (riscv*-*-*): Handle --with-isa-spec=.
	* config.in (HAVE_AS_MISA_SPEC): New.
	(HAVE_AS_MARCH_ZIFENCEI): Ditto.
	* config/riscv/riscv-opts.h (riscv_isa_spec_class): New.
	(riscv_isa_spec): Ditto.
	* config/riscv/riscv.h (HAVE_AS_MISA_SPEC): New.
	(ASM_SPEC): Pass -misa-spec if gas supported.
	* config/riscv/riscv.opt (riscv_isa_spec_class) New.
	* configure.ac (HAVE_AS_MARCH_ZIFENCEI): New test.
	(HAVE_AS_MISA_SPEC): Ditto.
	* configure: Regen.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-9.c: New.
	* gcc.target/riscv/arch-10.c: Ditto.
	* gcc.target/riscv/arch-11.c: Ditto.
	* gcc.target/riscv/attribute-6.c: Remove, we don't support G
	with version anymore.
	* gcc.target/riscv/attribute-8.c: Reorder arch string to fit canonical
	ordering.
	* gcc.target/riscv/attribute-9.c: We don't emit version for
	unknown extensions now.
	* gcc.target/riscv/attribute-11.c: Add -misa-spec=2.2 flags.
	* gcc.target/riscv/attribute-12.c: Ditto.
	* gcc.target/riscv/attribute-13.c: Ditto.
	* gcc.target/riscv/attribute-14.c: Ditto.
	* gcc.target/riscv/attribute-15.c: New.
	* gcc.target/riscv/attribute-16.c: Ditto.
	* gcc.target/riscv/attribute-17.c: Ditto.

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