RISC-V: Fix -march error of zhinxmin testcases

This little patch fixs the -march error of a zhinxmin testcase I added earlier
and an old zhinxmin testcase, since these testcases are for zhinxmin extension
and not zfhmin extension.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/_Float16-zhinxmin-3.c: Adjust.
	* gcc.target/riscv/_Float16-zhinxmin-4.c: Ditto.