Auto merge of rust-lang#3739 - joboet:macos_tls_dtors, r=RalfJung

Implement support for multiple TLS destructors on macOS

I want to get rid of [this `#[cfg]` block](https://github.com/rust-lang/rust/blob/98dcbae5c9ac615d5acfbf42d42b19a77cb9ec11/library/std/src/thread/mod.rs#L195-L211) in `std`, but it is currently required for miri, as it does not support multiple macOS TLS destructors. This is not true for the platform itself, however, as can be observed in the [implementation](https://github.com/apple-oss-distributions/dyld/blob/d552c40cd1de105f0ec95008e0e0c0972de43456/dyld/DyldRuntimeState.cpp#L2239).

Author: bors