emulate tls on android (below api 29) and cygwin build

[Chilledheart]

  /// Tests whether the target uses emulated TLS as default.
  ///
  /// Note: Android API level 29 (10) introduced ELF TLS.
  bool hasDefaultEmulatedTLS() const {
    return (isAndroid() && isAndroidVersionLT(29)) || isOSOpenBSD() ||
           isWindowsCygwinEnvironment() || isOHOSFamily();
  }

https://clang.llvm.org/docs/UsersManual.html#cmdoption-femulated-tls

Clang (and lld) do support native TLS, and mingw-w64 does have the things that are needed. I think binutils also might have what's needed too, but AFAIK the thing that's missing is support for it in GCC. Actually, (upstream) Clang defaults to native TLS instead of emulated TLS. In MSYS2, Clang is overridden to use emulated TLS by deafult to interoperate better with GCC built code and libstdc++ though.

https://news.ycombinator.com/item?id=29618539

[Chilledheart]

(gdb) b __emutls_get_address
Breakpoint 1 at 0x3d999c: file ./lib/builtins/emutls.c, line 393.