RISC-V: Emit costs for bool and stepped const vectors

RISC-V: Emit costs for bool and stepped const vectors

These cases are handled in the expander
(riscv-v.cc:expand_const_vector). We need the vector builder to detect
these cases so extract that out into a new riscv-v.h header file.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (class rvv_builder): Move to riscv-v.h.
	* config/riscv/riscv.cc (riscv_const_insns): Emit placeholder costs for
	bool/stepped const vectors.
	* config/riscv/riscv-v.h: New file.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>