rs6000: Redo darn (PR103624)

The builtins now all return "long".  The patterns have :GPR as the
output mode, so they can be 32-bit as well (the instruction makes sense
in 32 bit just fine).  The builtins expand to the DImode version
normally, but to the SImode if {32bit} is true.

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

	PR target/103624
	* config/rs6000/rs6000-builtins.def (__builtin_darn): Expand to
	darn_64_di.  Add {32bit} attribute.  Return long.
	(__builtin_darn_32): Expand to darn_32_di.  Add {32bit} attribute.
	Return long.
	(__builtin_darn_raw): Expand to darn_raw_di.  Add {32bit} attribute.
	Return long.
	* config/rs6000/rs6000-call.c (rs6000_expand_builtin): Expand the darn
	builtins to the _si variants for -m32.
	* config/rs6000/rs6000.md (UNSPECV_DARN_32, UNSPECV_DARN_RAW): Delete.
	(UNSPECV_DARN): Update comment.
	(darn_32, darn_raw, darn): Delete.
	(darn_32_<mode>, darn_64_<mode>, darn_raw_<mode> for GPR): New.
	(@darn<mode> for GPR): New.