Auto merge of #144465 - orlp:system-alloc-tls, r=Mark-Simulacrum

Allow the global allocator to use thread-local storage and std::thread::current()

Fixes rust-lang/rust#115209.

Currently the thread-local storage implementation uses the `Global` allocator if it needs to allocate memory in some places. This effectively means the global allocator can not use thread-local variables. This is a shame as an allocator is precisely one of the locations where you'd *really* want to use thread-locals. We also see that this lead to hacks such as rust-lang/rust#116402, where we detect re-entrance and abort.

So I've made the places where I could find allocation happening in the TLS implementation use the `System` allocator instead. I also applied this change to the storage allocated for a `Thread` handle so that it may be used care-free in the global allocator as well, for e.g. registering it to a central place or parking primitives.

r? `@joboet`

Author: bors