Throw an error when using custom reductions on non-Intel platforms (#473)

The error thrown by Julia when getting to closures unsupported on non-Intel
platforms is like this:

```
ERROR: LoadError: cfunction: closures are not supported on this platform
in expression starting at /path/to/file.jl:line
```

and nothing else, without a full stacktrace, but `/path/to/file.jl:line` is only
the top-level frame of the stack.  It can take a while to understand where the
error is actually coming from especially if several packages are involved,
provided that you're willing to debug it, otherwise users are out of luck.

This change makes the origin of the error clearer.

Author: giordano

src/operators.jl

@@ -69,12 +69,14 @@ end
 
 
 function Op(f, T=Any; iscommutative=false)
-    if MPI_LIBRARY == MicrosoftMPI && Sys.WORD_SIZE == 32
+    @static if MPI_LIBRARY == MicrosoftMPI && Sys.WORD_SIZE == 32
         error("User-defined reduction operators are not supported on 32-bit Windows.\nSee https://github.com/JuliaParallel/MPI.jl/issues/246 for more details.")
+    elseif Sys.ARCH ∈ (:aarch64, :ppc64le, :powerpc64le) || startswith(lowercase(String(Sys.ARCH)), "arm")
+        error("User-defined reduction operators are currently not supported on non-Intel architectures.\nSee https://github.com/JuliaParallel/MPI.jl/issues/404 for more details.")
     end
     w = OpWrapper{typeof(f),T}(f)
     fptr = @cfunction($w, Cvoid, (Ptr{Cvoid}, Ptr{Cvoid}, Ptr{Cint}, Ptr{MPI_Datatype}))
-    
+
     op = Op(OP_NULL.val, fptr)
     # int MPI_Op_create(MPI_User_function* user_fn, int commute, MPI_Op* op)
     @mpichk ccall((:MPI_Op_create, libmpi), Cint,
@@ -84,4 +86,3 @@ function Op(f, T=Any; iscommutative=false)
     finalizer(free, op)
     return op
 end
-