MFC r257877:
Don't try to use the 32-bit drti.o unless the data model is explicitly set
to ILP32. Otherwise dtrace -G will attempt to use it on amd64 if it can't
determine which data model to use, which happens when -64 is omitted and
no object files are provided, e.g. with

# dtrace -G -n BEGIN

This would result in a linker error, but now works properly.

Also remove an unnecessary #ifdef.