link.Elf.ZigObject.updateFunc: reduce data dependencies

Unfortunately it's not a complete solution, so a follow-up commit will
need to do something more drastic like not do the linker task queue at
the same time as codegen task queue.

From that point, it is possible to do more work at the same time but
that should be a separate branch. This one has gotten big enough.

src/codegen.zig

@@ -866,7 +866,7 @@ fn genNavRef(
             zo.symbol(sym_index).flags.is_extern_ptr = true;
             return .{ .mcv = .{ .lea_symbol = sym_index } };
         }
-        const sym_index = try zo.getOrCreateMetadataForNav(elf_file, nav_index);
+        const sym_index = try zo.getOrCreateMetadataForNav(zcu, nav_index);
         if (!single_threaded and is_threadlocal) {
             return .{ .mcv = .{ .load_tlv = sym_index } };
         }