- Implied rule are introduced into latest RISC-V ISA spec.

  - Only implemented D implied F-extension. Zicsr and Zifence are not
    implement yet, so the rule not included in this patch.

  - Pass preprocessed arch string to arch.

  - Verified with binutils 2.30 and 2.34.

gcc/ChangeLog

	* common/config/riscv/riscv-common.c (riscv_implied_info_t): New.
	(riscv_implied_info): New.
	(riscv_subset_list): Add handle_implied_ext.
	(riscv_subset_list::to_string): New parameter version_p to
	control output format.
	(riscv_subset_list::handle_implied_ext): New.
	(riscv_subset_list::parse_std_ext): Call handle_implied_ext.
	(riscv_arch_str): New parameter version_p to control output format.
	(riscv_expand_arch): New.
	* config/riscv/riscv-protos.h (riscv_arch_str): New parameter,
	version_p.
	* config/riscv/riscv.h (riscv_expand_arch): New,
	(EXTRA_SPEC_FUNCTIONS): Define.
	(ASM_SPEC): Transform -march= via riscv_expand_arch.

gcc/testsuite/ChangeLog

	* gcc.target/riscv/arch-6.c: New.
	* gcc.target/riscv/attribute-11.c: New.
	* gcc.target/riscv/attribute-12.c: New.