6941224: Improved stack overflow handling for Zero

Adding stack overflow checking to Shark brought to light a bunch of deficiencies in Zero's stack overflow code.

Reviewed-by: twisti

hotspot/src/cpu/zero/vm/cppInterpreter_zero.hpp

@@ -38,10 +38,6 @@
   // Main loop of normal_entry
   static void main_loop(int recurse, TRAPS);
 
- private:
-  // Stack overflow checks
-  static bool stack_overflow_imminent(JavaThread *thread);
-
  private:
   // Fast result type determination
   static BasicType result_type_of(methodOop method);