ISA string doesn't allow underscore separators

[Timmmm]

The constants.isa_regex requires the sub-extensions (Z..., S...) to immediately follow the main extensions without a separating underscore. For example this is considered valid:

RV32IMAFCSUZicsr_Zifencei_Zba_Zbb_Zbs

But this is not:

RV32IMAFCSU_Zicsr_Zifencei_Zba_Zbb_Zbs

Both are legal according to the spec. The latter is also easier to read and generate.