RISC-V: Add zifencei and prefixed h class extensions.

RISC-V: Add zifencei and prefixed h class extensions.

	bfd/
	* elfxx-riscv.c (riscv_parse_std_ext): Stop parsing standard
	extensions when parsed h keyword.
	(riscv_get_prefix_class): Support prefixed h class.
	(riscv_std_h_ext_strtab): Likewise.
	(riscv_ext_h_valid_p): Likewise.
	(parse_config): Likewise.
	(riscv_std_z_ext_strtab): Add zifencei.
	* elfxx-riscv.h (riscv_isa_ext_class): Add RV_ISA_CLASS_H.

	gas/
	* testsuite/gas/riscv/march-fail-order-z.d: New testcase, check
	orders of prefixed z extensions.
	* testsuite/gas/riscv/march-fail-order-z.l: Likewise.
	* testsuite/gas/riscv/march-fail-single-char-h.d: New testcase.
	* testsuite/gas/riscv/march-fail-single-char.l: Updated.
	* testsuite/gas/riscv/march-fail-unknown-h.d: New testcase.
	* testsuite/gas/riscv/march-fail-unknown.l: Updated.

	opcodes/
	* riscv-opc.c (riscv_ext_version_table): Add zifencei.