RISC-V: Refine register_builtin_types function. [NFC]

RISC-V: Refine register_builtin_types function. [NFC]

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins.cc (builtin_types): Redefine
	vector types.
	(build_const_pointer): New function.
	(register_builtin_type): Ditto.
	(DEF_RVV_TYPE): Simplify macro.
	(register_vector_type): Refine implementation.
	* config/riscv/riscv-vector-builtins.h (rvv_builtin_types_t): New.