[RISCV] Modify arch string parsing order according to latest riscv spec

According to latest risc-v spec, the canonical order in which extension names must appear in the name string specified in Table 29.1 is different from before. In the latest table, non-standard extensions must be listed after all standard extensions. To keep consistent, we now change the parsing order in parseArchString().

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D148315