8038212: Method::is_valid_method() check has performance regression impact for stackwalking

Only prune metaspace virtual spaces at safepoint so walking them is safe outside a safepoint.

Reviewed-by: mgerdin, mgronlun, hseigel, stefank

hotspot/src/share/vm/oops/method.hpp

@@ -861,6 +861,7 @@ class Method : public Metadata {
   const char* internal_name() const { return "{method}"; }
 
   // Check for valid method pointer
+  static bool has_method_vptr(const void* ptr);
   bool is_valid_method() const;
 
   // Verify