Add a jss gdb macro that skips over the jitted stack in gdb

tools/gdbinit

 # Copyright 2014 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Print HeapObjects.
 define job
 print ((v8::internal::HeapObject*)($arg0))->Print()
 end
 document job
 Print a v8 JavaScript object
@@ skipping 38 matching lines @@
 
 # Print JavaScript stack trace.
 define jst
 print v8::internal::Isolate::Current()->PrintStack((FILE*) stdout, 1)
 end
 document jst
 Print the current JavaScript stack trace
 Usage: jst
 end
 
+# Skip the JavaScript stack.
+define jss
+set $js_entry_sp=v8::internal::Isolate::Current()->thread_local_top()->js_entry_sp_
+set $rbp=*(void**)$js_entry_sp
+set $rsp=$js_entry_sp + 2*sizeof(void*)
+set $pc=*(void**)($js_entry_sp+sizeof(void*))
+end
+document jss
+Skip the jitted stack on x64 to where we entered JS last.
+Usage: jss
+end
+
 set disassembly-flavor intel
 set disable-randomization off