RISC-V: Handle zi* extension correctly for arch-canonicalize script

RISC-V: Handle zi* extension correctly for arch-canonicalize script

Canonical order for z-prefixed extension are rely on the canonical order of
single letter extension, however we didn't put i into the list before,
so when we put zicsr or zifencei it will got exception.

gcc/ChangeLog:

	* config/riscv/arch-canonicalize (CANONICAL_ORDER): Add `i` to
	CANONICAL_ORDER.