testsuite: riscv: set expected code model

testsuite: riscv: set expected code model

When testing a compiler configured to default to the medany code
model, riscv autovec binop vadd and vsub -nofm tests fail because
their codegen expectations don't match the generated code.

Set the code model to the medlow model that the tests expect.

for  gcc/testsuite/ChangeLog

	* gcc.target/riscv/rvv/autovec/binop/vadd-rv32gcv-nofm.c: Set the
	expected code model explicitly.
	* gcc.target/riscv/rvv/autovec/binop/vadd-rv64gcv-nofm.c: Likewise.
	* gcc.target/riscv/rvv/autovec/binop/vsub-rv32gcv-nofm.c: Likewise.
	* gcc.target/riscv/rvv/autovec/binop/vsub-rv64gcv-nofm.c: Likewise.