RISC-V: Move class riscv_subset_list and riscv_subset_t to riscv-protos.h

RISC-V: Move class riscv_subset_list and riscv_subset_t to riscv-protos.h

Pre-work of new style of architecture extension test macros, we need the
list used in `config/riscv/riscv-c.c`, so those struct/class declaration
must move to header file rather than local C file.

gcc/ChangeLog

	* common/config/riscv/riscv-common.c (RISCV_DONT_CARE_VERSION):
	Move to riscv-subset.h.
	(struct riscv_subset_t): Ditto.
	(class riscv_subset_list): Ditto.
	* config/riscv/riscv-subset.h (RISCV_DONT_CARE_VERSION): Move
	from riscv-common.c.
	(struct riscv_subset_t): Ditto.
	(class riscv_subset_list): Ditto.
	* config/riscv/t-riscv ($(common_out_file)): Add file
	dependency.