RISC-V: Add testcases for vector truncate after .SAT_SUB

RISC-V: Add testcases for vector truncate after .SAT_SUB

This patch would like to add the test cases of the vector truncate after
.SAT_SUB.  Aka:

  #define DEF_VEC_SAT_U_SUB_TRUNC_FMT_1(OUT_T, IN_T)                   \
  void __attribute__((noinline))                                       \
  vec_sat_u_sub_trunc_##OUT_T##_fmt_1 (OUT_T *out, IN_T *op_1, IN_T y, \
  				     unsigned limit)                 \
  {                                                                    \
    unsigned i;                                                        \
    for (i = 0; i < limit; i++)                                        \
      {                                                                \
        IN_T x = op_1[i];                                              \
        out[i] = (OUT_T)(x >= y ? x - y : 0);                          \
      }                                                                \
  }

The below 3 cases are included.

DEF_VEC_SAT_U_SUB_TRUNC_FMT_1(uint8_t, uint16_t)
DEF_VEC_SAT_U_SUB_TRUNC_FMT_1(uint16_t, uint32_t)
DEF_VEC_SAT_U_SUB_TRUNC_FMT_1(uint32_t, uint64_t)

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/vec_sat_arith.h: Add helper
	test macros.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_binary_scalar.h: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-1.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-2.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-3.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-run-1.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-run-2.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-run-3.c: New test.

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