RISC-V: Rename internal operand modifier N to n

RISC-V: Rename internal operand modifier N to n

Here is a purposal that using N for printing register encoding number,
so let rename the existing internal operand modifier `N` to `n`.

gcc/ChangeLog:

	* config/riscv/corev.md (*cv_branch<mode>): Update modifier.
	(*branch<mode>): Ditto.
	* config/riscv/riscv.cc (riscv_print_operand): Update modifier.
	* config/riscv/riscv.md (*branch<mode>): Update modifier.