Changes to cpp_rpc to make it work on Android (+ Hexagon offloading) (#5535)

Changes to cpp_rpc to make it work on Android (+ Hexagon offloading) (#5535)

* Changes to cpp_rpc to make it work on Android (+ Hexagon offloading)

- Implement getNextString to break up std::string into words. stringstream
  just doesn't work on Android.
- string::find_last_of doesn't look for the last substring, but the
  last character from a given string.
- Use SIGTERM to terminate processes (this isn't necessary, but using
  SIGKILL is not a good practice).
- Convert "./rpc" to a full path. When a module is uploaded and offloaded
  to Hexagon, the dlopen on Hexagon needs an absolute path (or a path
  without directories).

* Only set the absolute patch on non-Windows platforms

Windows has different macros for the maximum path length.