RISC-V: Support zicsr and zifencei extension for -march.

RISC-V: Support zicsr and zifencei extension for -march.

 - CSR related instructions and fence instructions has to be splitted from
   baseline ISA, zicsr and zifencei are corresponding sub-extension.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c (riscv_implied_info):
	d and f implied zicsr.
	(riscv_ext_flag_table): Handle zicsr and zifencei.
	* config/riscv/riscv-opts.h (MASK_ZICSR): New.
	(MASK_ZIFENCEI): Ditto.
	(TARGET_ZICSR): Ditto.
	(TARGET_ZIFENCEI): Ditto.
	* config/riscv/riscv.md (clear_cache): Check TARGET_ZIFENCEI.
	(fence_i): Ditto.
	* config/riscv/riscv.opt (riscv_zi_subext): New.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-8.c: New.
	* gcc.target/riscv/attribute-14.c: Ditto.