RISC-V: Point our Python scripts at python3

This builds for me, and I frequently have python-is-python3 type
packages installed so I think I've been implicitly testing it for a
while.  Looks like Kito's tested similar configurations, and the
bugzilla indicates we should be moving over.

gcc/ChangeLog:

	PR other/109668
	* config/riscv/arch-canonicalize: Move to python3
	* config/riscv/multilib-generator: Likewise