RISC-V: Add testcases for unsigned .SAT_SUB vector form 6

RISC-V: Add testcases for unsigned .SAT_SUB vector form 6

After the middle-end support the form 6 of unsigned SAT_SUB and
the RISC-V backend implement the .SAT_SUB for vector mode,  thus
add more test case to cover that.

Form 6:
  #define DEF_VEC_SAT_U_SUB_FMT_6(T)                                   \
  void __attribute__((noinline))                                       \
  vec_sat_u_sub_##T##_fmt_6 (T *out, T *op_1, T *op_2, unsigned limit) \
  {                                                                    \
    unsigned i;                                                        \
    for (i = 0; i < limit; i++)                                        \
      {                                                                \
        T x = op_1[i];                                                 \
        T y = op_2[i];                                                 \
        out[i] = x <= y ? 0 : x - y;                                   \
      }                                                                \
  }

Passed the rv64gcv regression test.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/vec_sat_arith.h: Add test macro.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub-21.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub-22.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub-23.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub-24.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub-run-21.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub-run-22.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub-run-23.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub-run-24.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>