**Issue**
It was annoying and error prone having to mirror these ifdefs around every function that could call these functions, and every function that could call those functions, etc, etc, etc. There were multiple compilation errors in aws-crt-cpp on iOS because we didn't exactly match the nested pattern of ifdefs previously seen here.

**Description of changes:**
Instead of ifdef-ing out platform-specific functions in the H file, just ifdef in the C file, and raise a runtime error on unsupported platforms.