Fix for 16 bit platforms (#12760) [backend]

This fixes some tiny issues with using Nim on 16-bit platforms. Not
entirely sure why the AVR chip I was compiling for with "cpu = avr" was
detected as 16-bit, but that's probably another issue..

lib/pure/math.nim

@@ -87,7 +87,9 @@ proc fac*(n: int): int =
     doAssert fac(4) == 24
     doAssert fac(10) == 3628800
   const factTable =
-    when sizeof(int) == 4:
+    when sizeof(int) == 2:
+      createFactTable[5]()
+    elif sizeof(int) == 4:
       createFactTable[13]()
     else:
       createFactTable[21]()