gdb: fix recent build breakage on 32 bit systems

The recent commit 3919cf8a70 caused a
build breakage on the 32-bit debian try bot. This seems related to how
the patch allocates space for the frame unwinder table in a gdbarch.
This commit changes that allocation to be in-place.

gdb/frame-unwind.c

@@ -73,14 +73,9 @@ static std::vector<const frame_unwind *> &
 get_frame_unwind_table (struct gdbarch *gdbarch)
 {
   std::vector<const frame_unwind *> *table = frame_unwind_data.get (gdbarch);
-  if (table != nullptr)
-    return *table;
-
-  table = new std::vector<const frame_unwind *>;
-  table->insert (table->begin (), standard_unwinders.begin (),
-		 standard_unwinders.end ());
-
-  frame_unwind_data.set (gdbarch, table);
+  if (table == nullptr)
+    table = frame_unwind_data.emplace (gdbarch, standard_unwinders.begin (),
+				       standard_unwinders.end ());
 
   return *table;
 }