testsuite: riscv: set expected code model for pr113206-2.c

testsuite: riscv: set expected code model for pr113206-2.c

With a compiler that defaults to cmodel medany, pr113206-2.c ends up
generating an extra vsetvli insn.

Set the code model explicitly to medlow to meet the expectation.

This is unfortunately not enough to get the test to a pass on trunk.
It is on gcc-15 and the change is an improvement, but trunk still gets
an unexpected vsetvli even with the change.

for  gcc/testsuite/ChangeLog

	* gcc.target/riscv/rvv/autovec/pr113206-2.c: Set the expected
	code model explicitly.