set has_thread_local=true for android

[name1e5s]

It seems that llvm uses emulated tls on android by default since this commit. Which uses a pthread_key to emulate various tls objects at runtime(code).

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.

By enable has_thread_local, we can reduce the number of pthread_keys used by rust libraries on android, which are quite limited resources(128 per process).

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⚠️ Warning ⚠️

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[est31]

cc rust-lang/compiler-builtins#458

[name1e5s]

Found that NDK actually links to __emutls_get_address in libgcc.a when build std without compiler-builtins-c feature on(the default behavior of prebuilt std crate). I think we'd better implement __emutls_get_address in compiler-builtins first, I'll do this when I have time.