RISC-V: Fix vector cfi notes for stack-clash protection

RISC-V: Fix vector cfi notes for stack-clash protection

The stack-clash code is generating wrong cfi directives in
riscv_v_adjust_scalable_frame because REG_CFA_DEF_CFA has a different
encoding than REG_FRAME_RELATED_EXPR, this patch fixes the offset sign
in prologue and starts using REG_CFA_DEF_CFA in the epilogue.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_v_adjust_scalable_frame): Add
	epilogue code for stack-clash and fix prologue cfi note.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/stack-check-cfa-3.c: Fix the expected output.