rs6000: Update darn testcases

Make the darn testcases work (and be tested) in 32-bit mode as well.
They used to ICE, but they no longer do.

2021-12-17  Segher Boessenkool <segher@kernel.crashing.org>

gcc/testsuite/
	PR target/103624
	* gcc.target/powerpc/darn-0.c: Remove target clause.
	* gcc.target/powerpc/darn-1.c: Remove target clause. Remove lp64
	requirement.  Change return type to long.
	* gcc.target/powerpc/darn-2.c: Ditto.
	* gcc.target/powerpc/darn-3.c: Remove target clause.