RISC-V: Support -misa-spec for arch-canonicalize and multilib-generator. [PR104853]

RISC-V: Support -misa-spec for arch-canonicalize and multilib-generator. [PR104853]

We migrate the default ISA spec version from 2.2 to 20191213, but those scripts
aren't updated at the same time, this patch is making both scripts support
different ISA spec versions.

gcc/ChangeLog:

	PR target/104853
	* config.gcc: Pass -misa-spec to arch-canonicalize and
	multilib-generator.
	* config/riscv/arch-canonicalize: Adding -misa-spec option.
	(SUPPORTED_ISA_SPEC): New.
	(arch_canonicalize): New argument `isa_spec`.
	Handle multiple ISA spec versions.
	* config/riscv/multilib-generator: Adding -misa-spec option.