RISC-V: Add testcases for scalar unsigned SAT_SUB form 4

RISC-V: Add testcases for scalar unsigned SAT_SUB form 4

After the middle-end support the form 4 of unsigned SAT_SUB and
the RISC-V backend implement the scalar .SAT_SUB, add more test
case to cover the form 4 of unsigned .SAT_SUB.

Form 4:
  #define SAT_SUB_U_4(T) \
  T sat_sub_u_4_##T (T x, T y) \
  { \
    return x >= y ? x - y : 0; \
  }

Passed the rv64gcv fully regression test.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/sat_arith.h: Add helper macro for test.
	* gcc.target/riscv/sat_u_sub-13.c: New test.
	* gcc.target/riscv/sat_u_sub-14.c: New test.
	* gcc.target/riscv/sat_u_sub-15.c: New test.
	* gcc.target/riscv/sat_u_sub-16.c: New test.
	* gcc.target/riscv/sat_u_sub-run-13.c: New test.
	* gcc.target/riscv/sat_u_sub-run-14.c: New test.
	* gcc.target/riscv/sat_u_sub-run-15.c: New test.
	* gcc.target/riscv/sat_u_sub-run-16.c: New test.

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