SafeStack: Create the unsafe stack pointer on demand.

This avoids creating an unnecessary undefined reference on targets such as
NVPTX that require such references to be declared in asm output.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240321 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/SafeStack/no-attr.ll

@@ -6,6 +6,8 @@
 ; no safestack attribute
 ; Requires no protector.
 
+; CHECK-NOT: __safestack_unsafe_stack_ptr
+
 ; CHECK: @foo
 define void @foo(i8* %a) nounwind uwtable {
 entry: