riscv: Prepare backend for index registers

riscv: Prepare backend for index registers

RISC-V does currently not support index registers.
However, there are some vendor extensions that specify them.
Let's do the necessary changes in the backend so that we can
add support for such a vendor extension in the future.

This is a non-functional change without any intended side-effects.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_regno_ok_for_index_p):
	New prototype.
	(riscv_index_reg_class): Likewise.
	* config/riscv/riscv.cc (riscv_regno_ok_for_index_p): New function.
	(riscv_index_reg_class): New function.
	* config/riscv/riscv.h (INDEX_REG_CLASS): Call new function
	riscv_index_reg_class().
	(REGNO_OK_FOR_INDEX_P): Call new function
	riscv_regno_ok_for_index_p().

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>