RISC-V: Small stack tie changes

Enable the register used by riscv_emit_stack_tie () to be passed as
an argument so we can tie the stack with other registers besides
hard_frame_pointer_rtx.
Also don't allow operand 1 of stack_tie<mode> to be optimized to sp
in preparation for the stack clash protection support.

gcc/ChangeLog:
	* config/riscv/riscv.cc (riscv_emit_stack_tie): Pass the
	register to be tied to the stack pointer as argument.
	* config/riscv/riscv.md (stack_tie<mode>): Don't match equal
	operands.