RISC-V: Fix one potential test failure for RVV vsetvl

RISC-V: Fix one potential test failure for RVV vsetvl

The test will fail on below command with multi-thread like below.  However,
it comes from one missed "Oz" option when check vsetvl.

make -j $(nproc) report RUNTESTFLAGS="rvv.exp riscv.exp"

To some reason, this failure cannot be reproduced by RUNTESTFLAGS="rvv.exp"
or make without -j option. We would like to fix it and root cause the
reason later.

Signed-off-by: Pan Li <pan2.li@intel.com>
gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/vsetvl/vsetvl-23.c: Adjust test checking.