fix RISC-V _Z extension parsing, they don't have a defined order

[camel-cdr]

I noticed that the current RISC-V ISA string parsing code assumes there is an order for the extensions. While this is true for single letter extensions, the others, e.g. with _Z prefix, don't have a defined order.

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[camel-cdr]

How do I sign the CLA without using an google account?