RISC-V: Add testcases for form 6 of vector signed SAT_TRUNC

Form 6:
  #define DEF_VEC_SAT_S_TRUNC_FMT_6(NT, WT, NT_MIN, NT_MAX)             \
  void __attribute__((noinline))                                        \
  vec_sat_s_trunc_##NT##_##WT##_fmt_6 (NT *out, WT *in, unsigned limit) \
  {                                                                     \
    unsigned i;                                                         \
    for (i = 0; i < limit; i++)                                         \
      {                                                                 \
        WT x = in[i];                                                   \
        NT trunc = (NT)x;                                               \
        out[i] = (WT)NT_MIN >= x || x > (WT)NT_MAX                      \
	  ? x < 0 ? NT_MIN : NT_MAX                                     \
	  j: trunc;                                                      \
      }                                                                 \
  }

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/unop/vec_sat_s_trunc-6-i16-to-i8.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-6-i32-to-i16.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-6-i32-to-i8.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-6-i64-to-i16.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-6-i64-to-i32.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-6-i64-to-i8.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-run-6-i16-to-i8.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-run-6-i32-to-i16.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-run-6-i32-to-i8.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-run-6-i64-to-i16.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-run-6-i64-to-i32.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/vec_sat_s_trunc-run-6-i64-to-i8.c: New test.

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