RISC-V: Add support for -mcpu option.

 - The behavior of -mcpu basically equal to -march plus -mtune, but it
   has lower priority than -march and -mtune.

 - The behavior and available options has sync with clang except we don't add
   few LLVM specific value, and add more sifive processor to the list.

 - -mtune also accept all available options of -mcpu, and use it setting.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c (riscv_cpu_tables): New.
	(riscv_arch_str): Return empty string if current_subset_list
	is NULL.
	(riscv_find_cpu): New.
	(riscv_handle_option): Verify option value of -mcpu.
	(riscv_expand_arch): Using std::string.
	(riscv_default_mtune): New.
	(riscv_expand_arch_from_cpu): Ditto.
	* config/riscv/riscv-cores.def: New.
	* config/riscv/riscv-protos.h (riscv_find_cpu): New.
	(riscv_cpu_info): New.
	* config/riscv/riscv.c (riscv_tune_info): Rename ...
	(riscv_tune_param): ... to this.
	(riscv_cpu_info): Rename ...
	(riscv_tune_info): ... to this.
	(tune_info): Rename ...
	(tune_param): ... to this.
	(rocket_tune_info): Update data type name.
	(sifive_7_tune_info): Ditto.
	(optimize_size_tune_info): Ditto.
	(riscv_cpu_info_table): Rename ...
	(riscv_tune_info_table): ... to this.
	(riscv_parse_cpu): Rename ...
	(riscv_parse_tune): ... to this, and translate valid -mcpu option to
	-mtune option.
	(riscv_rtx_costs): Rename tune_info to tune_param.
	(riscv_class_max_nregs): Ditto.
	(riscv_memory_move_cost): Ditto.
	(riscv_init_machine_status): Use value of -mcpu if -mtune is not
	given, and rename tune_info to tune_param.
	* config/riscv/riscv.h (riscv_expand_arch_from_cpu): New.
	(riscv_default_mtune): Ditto.
	(EXTRA_SPEC_FUNCTIONS): Add riscv_expand_arch_from_cpu and
	riscv_default_mtune.
	(OPTION_DEFAULT_SPECS): Handle default value of -march/-mabi.
	(DRIVER_SELF_SPECS): Expand -march from -mcpu if -march is not
	given.
	* config/riscv/riscv.opt (-mcpu): New option.
	* config/riscv/t-riscv ($(common_out_file)): Add
	riscv-cores.def to dependency.
	* doc/invoke.texi (RISC-V Option): Add -mcpu, and update the
	description of default value for -mtune and -march.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/mcpu-1.c: New.
	* gcc.target/riscv/mcpu-2.c: Ditto.
	* gcc.target/riscv/mcpu-3.c: Ditto.
	* gcc.target/riscv/mcpu-4.c: Ditto.
	* gcc.target/riscv/mcpu-5.c: Ditto.
	* gcc.target/riscv/mcpu-6.c: Ditto.
	* gcc.target/riscv/mcpu-7.c: Ditto.

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