now can generate both targets for npvtx64. m_isPTX is set true, to distuish when to either skip or exlcusive euse export

ispc.h

@@ -189,7 +189,7 @@ public:
     /** Initializes the given Target pointer for a target of the given
         name, if the name is a known target.  Returns true if the
         target was initialized and false if the name is unknown. */
-    Target(const char *arch, const char *cpu, const char *isa, bool pic);
+    Target(const char *arch, const char *cpu, const char *isa, bool pic, bool isPTX = false);
 
     /** Returns a comma-delimited string giving the names of the currently
         supported compilation targets. */