Don't use __attribute__((noreturn)) on VxWorks

VxWorks does not mark abort() or exit() as noreturn.
Thus, functions declared noreturn which end in a call
to those functions cause a compiler warning on vxWorks.

modules/libcom/src/osi/compiler/gcc/compilerSpecific.h

@@ -60,6 +60,9 @@
 /*
  * No return marker
  */
+#ifndef vxWorks
+// VxWorks does not mark abort() or exit() noreturn!
 #define EPICS_NORETURN __attribute__((noreturn))
+#endif
 
 #endif  /* ifndef compilerSpecific_h */