RISC-V: Add testcases for form 2 of vector signed SAT_SUB

Form 2:
  #define DEF_VEC_SAT_S_SUB_FMT_2(T, UT, MIN, MAX)                     \
  void __attribute__((noinline))                                       \
  vec_sat_s_sub_##T##_fmt_2 (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];                                                 \
        T minus = (UT)x - (UT)y;                                       \
        out[i] = (x ^ y) >= 0 || (minus ^ x) >= 0                      \
          ? minus : x < 0 ? MIN : MAX;                                 \
      }                                                                \
  }

DEF_VEC_SAT_S_SUB_FMT_2(int8_t, uint8_t, INT8_MIN, INT8_MAX)

The below test are passed for this patch.
* The rv64gcv fully regression test.

It is test only patch and obvious up to a point, will commit it
directly if no comments in next 48H.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vec_sat_arith.h: Add test helper macros.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_s_sub-2-i16.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_s_sub-2-i32.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_s_sub-2-i64.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_s_sub-2-i8.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_s_sub-run-2-i16.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_s_sub-run-2-i32.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_s_sub-run-2-i64.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_s_sub-run-2-i8.c: New test.

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