Add the "no-split-stack" attribute

This disable emission of segmented-stack prologues on a per-function basis, even
if segmented stacks are turned on globally.

lib/CodeGen/PrologEpilogInserter.cpp

@@ -680,7 +680,8 @@ void PEI::insertPrologEpilogCode(MachineFunction &Fn) {
   // we've been asked for it.  This, when linked with a runtime with support
   // for segmented stacks (libgcc is one), will result in allocating stack
   // space in small chunks instead of one large contiguous block.
-  if (Fn.getTarget().Options.EnableSegmentedStacks)
+  if (Fn.getTarget().Options.EnableSegmentedStacks &&
+      !Fn.getFunction()->hasFnAttribute("no-split-stack"))
     TFI.adjustForSegmentedStacks(Fn);
 
   // Emit additional code that is required to explicitly handle the stack in