Fix broken TLS destructors on 32-bit win7
Fixesrust-lang/rust#141300
On the 32-bit win7 target, we use OS TLS instead of native TLS, due to issues with how the OS handles alignment. Unfortunately, this caused issues due to the TLS destructors not running, causing memory leaks among other problems.
On Windows, to support OS TLS, the TlsAlloc family of function is used by Rust. This function does not support TLS destructors at all. However, rust has some code to emulate those destructors, by leveraging the TLS support functionality found in the MSVC CRT (specifically, in tlssup.c of the CRT).
To use this functionality, the user must do two things:
1. They must put the address to their callback in a section between `.CRT$XLB` and `.CRT$XLY`.
2. They must add a reference to `_tls_used` (or `__tls_used` on x86) to make sure the TLS support code in tlssup.c isn't garbage collected by the linker.
Prior to this commit, this second bit wasn't being done properly by the Rust TLS support code. Instead of adding a reference to _tls_used, it instead had a reference to its own callback to prevent it from getting GC'd by the linker. While this is _also_ necessary, not having a reference on _tls_used made the entire support non-functional.
This commit reworks the code to:
1. Add an unconditional `#[used]` attribute on the CALLBACK, which should be enough to prevent it from getting GC'd by the linker.
2. Add a reference to `_tls_used`, which should pull the TLS support code into the Rust programs and not let it be GC'd by the linker.
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