do not insert a prototype for malloc of: void* malloc(uint): on 64-bit u
targets this is not correct.  Instead of prototype it as void *malloc(...),
and pass the correct intptr_t through the "...".

Finally, fix Regression/CodeGen/SparcV9/2004-12-13-MallocCrash.ll, by not
forming constantexpr casts from pointer to uint.

llvm-svn: 18908