set has_thread_local=true for android It seems that llvm uses emulated tls on android by default since [this commit](https://reviews.llvm.org/D42999). Which uses a `pthread_key` to emulate various tls objects at runtime([code](https://github.com/llvm-mirror/compiler-rt/blob/master/lib/builtins/emutls.c)). I've built a demo with a custom android target setting `has_thread_local=true` and a slightly modified `std` as what this PR does, and it works fine. The generated function symbol `__emutls_get_address` would link to an implementation in `libgcc.a` , which is also [required by std](https://github.com/rust-lang/rust/blob/master/library/unwind/build.rs#L18) for now. By enable `has_thread_local`, we can reduce the number of `pthread_key`s used by rust libraries on android, which are quite limited resources(128 per process). I've been investgating some crashes caused by unable to create more pthread_key in our project with about 80 `pthread_key`s used by rust part. cc rust-lang#96145 [rust-lang/compiler-builtins#458](rust-lang/compiler-builtins#458)

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