riscv: Restructure callee-saved register save/restore code

riscv: Restructure callee-saved register save/restore code

This patch restructures the loop over the GP registers
which saves/restores then as part of the prologue/epilogue.
No functional change is intended by this patch, but it
offers the possibility to use load-pair/store-pair instructions.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_next_saved_reg): New function.
	(riscv_is_eh_return_data_register): New function.
	(riscv_for_each_saved_reg): Restructure loop.

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