[RISC-V] correct machine mode in save-restore cfi RTL.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_adjust_libcall_cfi_prologue): Use Pmode
	for cfi reg/mem machmode
	(riscv_adjust_libcall_cfi_epilogue): Use Pmode for cfi reg machmode

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/save-restore-cfi-2.c: New test to check machmode
	for cfi reg/mem.