RISC-V: Explicitly call python when using multilib generator

RISC-V: Explicitly call python when using multilib generator

When building GCC for RISC-V with the --with-multilib-generator option,
it may not be possible to call arch-canonicalize as an executable when
building on Windows. Instead directly invoke the expected python
interpreter for this step.

gcc/ChangeLog:

	* config/riscv/multilib-generator (arch_canonicalize): Invoke
	python interpreter when calling arch-canonicalize script.