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.

gcc/config/riscv/multilib-generator

@@ -54,7 +54,8 @@ def arch_canonicalize(arch):
   this_file = os.path.abspath(os.path.join( __file__))
   arch_can_script = \
     os.path.join(os.path.dirname(this_file), "arch-canonicalize")
-  proc = subprocess.Popen([arch_can_script, arch], stdout=subprocess.PIPE)
+  proc = subprocess.Popen([sys.executable, arch_can_script, arch],
+                          stdout=subprocess.PIPE)
   out, err = proc.communicate()
   return out.strip()