RISC-V: Allow extension name contain digit

RISC-V: Allow extension name contain digit

RISC-V spec only allow alphabetical name for extension before, however
vector extension add several extension named with digits, so we try to
extend the naming rule.

Ref:
https://github.com/riscv/riscv-isa-manual/pull/718

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c
	(riscv_subset_list::parse_multiletter_ext): Allow ext. name has
	digit.