win64,stackwalk: remove support for binaries missing SEH info

This hack, added in 7e95d09, was a
workaround for LLVM3.3 not being able to generate binaries with the
mandatory SEH section information. Now, it is just a potential source of
segfaults in observation.

src/stackwalk.c

@@ -358,6 +358,7 @@ JL_DLLEXPORT jl_value_t *jl_get_excstack(jl_task_t* task, int include_bt, int ma
 }
 
 #if defined(_OS_WINDOWS_)
+// XXX: these caches should be per-thread
 #ifdef _CPU_X86_64_
 static UNWIND_HISTORY_TABLE HistoryTable;
 #else
@@ -495,14 +496,9 @@ static int jl_unw_step(bt_cursor_t *cursor, uintptr_t *ip, uintptr_t *sp)
 
  PRUNTIME_FUNCTION FunctionEntry = (PRUNTIME_FUNCTION)JuliaFunctionTableAccess64(
  GetCurrentProcess(), cursor->Rip);
- if (!FunctionEntry) { // assume this is a NO_FPO RBP-based function
- cursor->Rsp = cursor->Rbp; // MOV RSP, RBP
- if (!readable_pointer((LPCVOID)cursor->Rsp))
- return 0;
- cursor->Rbp = *(DWORD64*)cursor->Rsp; // POP RBP
- cursor->Rsp += sizeof(void*);
- cursor->Rip = *(DWORD64*)cursor->Rsp; // POP RIP (aka RET)
- cursor->Rsp += sizeof(void*);
+ if (!FunctionEntry) {
+ // Not code or bad unwind?
+ return 0;
  }
  else {
  PVOID HandlerData;