RISC-V: Add testcases for scalar unsigned SAT_SUB form 6

RISC-V: Add testcases for scalar unsigned SAT_SUB form 6

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

Form 6:
  #define SAT_SUB_U_6(T) \
  T sat_sub_u_6_##T (T x, T y) \
  { \
    return x <= y ? 0 : x - y; \
  }

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/sat_arith.h: Add helper macro for test.
	* gcc.target/riscv/sat_u_sub-21.c: New test.
	* gcc.target/riscv/sat_u_sub-22.c: New test.
	* gcc.target/riscv/sat_u_sub-23.c: New test.
	* gcc.target/riscv/sat_u_sub-24.c: New test.
	* gcc.target/riscv/sat_u_sub-run-21.c: New test.
	* gcc.target/riscv/sat_u_sub-run-22.c: New test.
	* gcc.target/riscv/sat_u_sub-run-23.c: New test.
	* gcc.target/riscv/sat_u_sub-run-24.c: New test.

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