RISC-V: Adjust the parsing order of extensions to be consistent with riscv-spec and...

RISC-V: Adjust the parsing order of extensions to be consistent with riscv-spec and binutils.

The current order of gcc and binutils parsing extensions is inconsistent.

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.

Related llvm patch links:
 https://reviews.llvm.org/D148315

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (multi_letter_subset_rank): Swap the order
	of z-extensions and s-extensions.
	(riscv_subset_list::parse): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-5.c: Likewise.