RISC-V: Sync extension of canonical order with spec

The canonical order has been adjusted a little bit [1-2].  Fortunately, the
affected extensions haven't been ratified yet, so there's no real impact.
Although P doesn't have an upstream yet, to avoid forgetting to update the
canonical order in the future, I've decided to update it now while I still
remember it.

[1] https://github.com/riscv/riscv-isa-manual/pull/2903
[2] https://github.com/riscv/riscv-isa-manual/pull/2910

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_supported_std_ext):
	Move 'v' before 'p' and drop 'n' to match the spec.
	* config/riscv/arch-canonicalize (CANONICAL_ORDER): Ditto.