rustc: Add feature-gated cfg(target_thread_local)

Currently the standard library has some pretty complicated logic to detect
whether #[thread_local] should be used or whether it's supported. This is also
unfortunately not quite true for OSX where not all versions support
the #[thread_local] attribute (only 10.7+ does). Compiling code for OSX 10.6 is
typically requested via the MACOSX_DEPLOYMENT_TARGET environment variable (e.g.
the linker recognizes this), but the standard library unfortunately does not
respect this.

This commit updates the compiler to add a `target_thread_local` cfg annotation
if the platform being targeted supports the `#[thread_local]` attribute. This is
feature gated for now, and it is only true on non-aarch64 Linux and 10.7+ OSX
(e.g. what the module already does today). Logic has also been added to parse
the deployment target environment variable.

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