RISC-V: Canonicalize --with-arch

RISC-V: Canonicalize --with-arch

 - We would like to canonicalize the arch string for --with-arch for
   easier handling multilib, so split canonicalization part to a stand
   along script to shared the logic.

gcc/ChangeLog:

	* config/riscv/multilib-generator (arch_canonicalize): Move
	code to arch-canonicalize, and call that script to canonicalize arch
	string.
	(canonical_order): Move code to arch-canonicalize.
	(LONG_EXT_PREFIXES): Ditto.
	(IMPLIED_EXT): Ditto.
	* config/riscv/arch-canonicalize: New.
	* config.gcc (riscv*-*-*): Canonicalize --with-arch.