RISC-V: Update march parser

RISC-V: Update march parser

 - The arch string rule has changed in latest spec, it introduced new
   multi-letter extension prefix with 'h' and 'z', and drop `sx`. also
   adjust parsing order for 's' and 'x'.

gcc/ChangeLog

	* riscv-common.c (parse_sv_or_non_std_ext): Rename to
	parse_multiletter_ext.
	(parse_multiletter_ext): Add parsing `h` and `z`, drop `sx`,
	adjust parsing order for 's' and 'x'.

gcc/testsuite/ChangeLog

	* gcc.target/riscv/arch-3.c: Adjust option.
	* gcc.target/riscv/arch-5.c: New.
	* gcc.target/riscv/attribute-9.c: Adjust option and test
	condition.