C++11 TLS simulation

[DNikolaevAtRocket]

The PR is re-targeted ibmruntimes#111.

The PR implements TLS simulation in C++11 addressing ibmruntimes#108.
It's supposed to be functionally complete.

Another goal is to make porting of C++11 code (or higher levels) using thread_local as seamless as possible.

Example:

// Original:
// thread_local std::string name = "default";

// Simulation:
static __tls<std::string> tls_name;
auto &name = tls_name.get([]{ return std::string("default"); });

// ...
name = "updated";

Thanks!