Pass -femulated-tls to the compiler by default

The MinGW CRT wrapping has a few minor issues that makes native
TLS fail at the moment.

LLVM doesn't support native TLS on windows/aarch64, but emulated
TLS is supported now.

README.md

@@ -68,7 +68,6 @@ normal GCC/binutils based MinGW.
   the dllimport attributes.
 - The C++ libraries ([libcxxabi](http://libcxxabi.llvm.org/), [libcxx](http://libcxx.llvm.org/)) can only be linked statically
   at the moment.
-- The ARM64 target doesn't support thread local variables.
 
 Additionally, one may run into other minor differences between GCC and clang.