Remove platform-specific (POSIX, Win32, gcc builtins, etc.) implementations of the ICU atomic and mutex functions, switching unconditionally to the use of C++11.
This does not include changing call sites within ICU that use the internal synchronization wrapper functions.
The motivation is that much of the platform-specific wrapper code is old, difficult to understand, and in need of maintenance - it has not kept up with improvements to the underlying platforms. Best to be rid of it.