std: Flag Windows TLS dtor symbol as #[used]

Turns out ThinLTO was internalizing this symbol and eliminating it. Worse yet if
you compiled with LTO turns out no TLS destructors would run on Windows! The
`#[used]` annotation should be a more bulletproof implementation (in the face of
LTO) of preserving this symbol all the way through in LLVM and ensuring it makes
it all the way to the linker which will take care of it.

src/libstd/lib.rs

@@ -332,6 +332,7 @@
 #![feature(doc_spotlight)]
 #![cfg_attr(test, feature(update_panic_count))]
 #![cfg_attr(windows, feature(const_atomic_ptr_new))]
+#![cfg_attr(windows, feature(used))]
 
 #![default_lib_allocator]