RISC-V: Extend syntax for the multilib-generator

RISC-V: Extend syntax for the multilib-generator

 - Support expansion operator (*) in the multilib config string.

 - Motivation of this patch is reduce the complexity when we deal multilib with
   sub-extension, expand the combinations by hand would be very painful and
   error prone, no one deserve to experience this[1] again!

[1] https://github.com/sifive/freedom-tools/blob/f4d7facafb27d16125768c90ff1790c674e4be7a/Makefile#L348

gcc/ChangeLog:

	* config/riscv/multilib-generator: Add TODO, import itertools
	and functools.reduce.
	Handle expantion operator.
	(LONG_EXT_PREFIXES): New.
	(arch_canonicalize): Update comment and improve python3
	debuggability/compatibility.
	(add_underline_prefix): New.
	(_expand_combination): Ditto.
	(unique): Ditto.
	(expand_combination): Ditto.