riscv: xtheadmempair: Fix CFA reg notes

riscv: xtheadmempair: Fix CFA reg notes

The current implementation triggers an assertion in
dwarf2out_frame_debug_cfa_offset() under certain circumstances.
The standard code uses REG_FRAME_RELATED_EXPR notes instead
of REG_CFA_OFFSET notes when saving registers on the stack.
So let's do this as well.

gcc/ChangeLog:

	* config/riscv/thead.cc (th_mempair_save_regs):
	Emit REG_FRAME_RELATED_EXPR notes in prologue.

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