[RISCV] Use consume_front to parse rv32/rv64 in RISCVISAInfo::parse*ArchString. NFC (#90562)

This replaces some starts_with calls wth consume_front. This allows us
to remove a later assumption that prefix was 4 characters. We would
eventually need to fix this anyway if we ever support rv128.

Noticed while reviewing the RISCVISAInfo code for other reasons.