RISC-V: Add test case of unsigned scalar SAT_MUL form 5 for widen-mul

RISC-V: Add test case of unsigned scalar SAT_MUL form 5 for widen-mul

The form 5 of unsigned scalar SAT_MUL is covered in middle-expand
alreay, add test case here to cover form 5.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/sat/sat_arith.h: Add test helper macros.
	* gcc.target/riscv/sat/sat_u_mul-6-u16-from-u128.c: New test.
	* gcc.target/riscv/sat/sat_u_mul-6-u32-from-u128.c: New test.
	* gcc.target/riscv/sat/sat_u_mul-6-u64-from-u128.c: New test.
	* gcc.target/riscv/sat/sat_u_mul-6-u8-from-u128.c: New test.
	* gcc.target/riscv/sat/sat_u_mul-run-6-u16-from-u128.c: New test.
	* gcc.target/riscv/sat/sat_u_mul-run-6-u32-from-u128.c: New test.
	* gcc.target/riscv/sat/sat_u_mul-run-6-u64-from-u128.c: New test.
	* gcc.target/riscv/sat/sat_u_mul-run-6-u8-from-u128.c: New test.

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