PR target/98152: Checking python is available before using

PR target/98152: Checking python is available before using

We'll try to canonicalize the arch string for --with-arch,
and the script is written in python, however it will turns out
GCC require python to build for RISC-V port, it's not expect as
the GCC requirement.

So this patch is made this as optional, detect python and only use it
when it available, it won't break any functionality with out doing
canonicalization, just might build one more redundant multi-lib.

gcc/ChangeLog:

	PR target/98152
	* config.gcc (riscv*-*-*): Checking python, python3 or python2
	is available, and skip doing with_arch canonicalize if no python
	available.