Add missing _cast_uitofp/fptoui, setzero and min_varying functions

This commit fixes several compfails (listed in fail_db.txt). No new runfails were noticed, but there are load/store functions which were compfailing and now might be runfailing with a segfault due to unaligned memory access.
This function was neither implemented nor was it used (by the code generator or knc.h itself). Why keep it?