[RISCV] Don't crash if parseNormalizedArchString encounters a multi-letter...

[RISCV] Don't crash if parseNormalizedArchString encounters a multi-letter extension with an unknown prefix. (#91398)

The sorting code previously asserted if a prefix was multiple letters,
but didn't start with s, x, or z.

Replace the assert with an explicit check and sort the multi-letter
extension after the known multi-letter prefixes.