Use different allocation when memory can not contain pointers.

R=iant
https://golang.org/cl/1849044

go/unsafe.cc

diff --git a/go/unsafe.cc b/go/unsafe.cc
index 437829f..51d812b 100644
--- a/go/unsafe.cc
+++ b/go/unsafe.cc
@@ -18,9 +18,6 @@
 {
   location_t bloc = BUILTINS_LOCATION;
 
-  // Importing the "unsafe" package automatically disables TBAA.
-  flag_strict_aliasing = false;
-
   bool add_to_globals;
   Package* package = this->add_imported_package("unsafe", local_name,
 						is_local_name_exported,
@@ -132,4 +129,6 @@
   no = bindings->add_function_declaration("NewArray", package, fntype, bloc);
   if (add_to_globals)
     this->add_named_object(no);
+
+  this->imported_unsafe_ = true;
 }