No, the GC stack is not in fact a constant

Fixes remaining MCJIT issues on OS X. I'm not exactly sure what LLVM did
in this case, but I know it can't be good. I'm actually surprised this works
some of the time. You wouldn't believe how much time I've spent tracking down
this one misplaced bit!

src/cgutils.cpp

@@ -8,7 +8,7 @@ static GlobalVariable *prepare_global(GlobalVariable *G)
  GlobalVariable *gv = jl_Module->getGlobalVariable(G->getName());
  if (!gv) {
  gv = new GlobalVariable(*jl_Module, G->getType()->getElementType(),
- true, GlobalVariable::ExternalLinkage,
+ G->isConstant(), GlobalVariable::ExternalLinkage,
  NULL, G->getName());
  }
  return gv; 

src/codegen.cpp

@@ -3646,7 +3646,7 @@ static void init_julia_llvm_env(Module *m)
 #ifdef JL_GC_MARKSWEEP
  jlpgcstack_var =
  new GlobalVariable(*m, jl_ppvalue_llvmt,
- true, GlobalVariable::ExternalLinkage,
+ false, GlobalVariable::ExternalLinkage,
  NULL, "jl_pgcstack");
  add_named_global(jlpgcstack_var, (void*)&jl_pgcstack);
 #endif