Use the native tls implementation on android

[alexcrichton]

Turns out android doesn't support LLVM's thread_local attribute and accompanying
implementation.

Closes #10686

[yichoi]

@alexcrichton could you check it again ?

/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local.rs:135:12: 135:35 error: unresolved name `local_ptr::init_tls_key`.
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local.rs:135             local_ptr::init_tls_key();
                                                                                                          ^~~~~~~~~~~~~~~~~~~~~~~
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local.rs:147:12: 147:35 error: unresolved name `local_ptr::init_tls_key`.
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local.rs:147             local_ptr::init_tls_key();
                                                                                                          ^~~~~~~~~~~~~~~~~~~~~~~
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local.rs:164:12: 164:35 error: unresolved name `local_ptr::init_tls_key`.
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local.rs:164             local_ptr::init_tls_key();
                                                                                                          ^~~~~~~~~~~~~~~~~~~~~~~
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local.rs:180:12: 180:35 error: unresolved name `local_ptr::init_tls_key`.
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local.rs:180             local_ptr::init_tls_key();
                                                                                                          ^~~~~~~~~~~~~~~~~~~~~~~
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local_ptr.rs:269:28: 269:74 error: unresolved name
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local_ptr.rs:269             cast::transmute(realstd::rt::shouldnt_be_public::maybe_tls_key())
                                                                                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local_ptr.rs:269:28: 269:74 error: use of undeclared module `realstd::rt::shouldnt_be_public`
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local_ptr.rs:269             cast::transmute(realstd::rt::shouldnt_be_public::maybe_tls_key())
                                                                                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local_ptr.rs:269:28: 269:74 error: unresolved name `realstd::rt::shouldnt_be_public::maybe_tls_key`.
/home/rustbuild/src/rust-buildbot/slave/auto-linux-64-nopt-t/build/src/libstd/rt/local_ptr.rs:269             cast::transmute(realstd::rt::shouldnt_be_public::maybe_tls_key())
                                                                                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to 7 previous errors