RISC-V: Split arguments of expand_vec_perm

Since the following patch will calls expand_vec_perm with
splitted arguments, change the expand_vec_perm interface in
this patch.

gcc/ChangeLog:

	* config/riscv/autovec.md: Split arguments.
	* config/riscv/riscv-protos.h (expand_vec_perm): Ditto.
	* config/riscv/riscv-v.cc (expand_vec_perm): Ditto.