Target handling fixes.

Set the Module's target appropriately when it's first created.
Compile separate 32 and 64 bit versions of the builtins-c bitcocde
  and load the appropriate one based on the target we're compiling
  for.

ispc.h

@@ -168,6 +168,9 @@ struct Target {
     /** Target system architecture.  (e.g. "x86-64", "x86"). */
     std::string arch;
 
+    /** Is the target architecture 32 or 64 bit */
+    bool is32bit;
+
     /** Target CPU. (e.g. "corei7", "corei7-avx", ..) */
     std::string cpu;