Split-off POSIX sync version into a separate file

[ethouris]

Recommended review:

1. Compare existing files (filelist.maf and sync.cpp)
2. Compare old sync.cpp against the new sync_posix.cpp: git difftool HEAD:sync.cpp sync_posix.cpp

Expected:

1. Updated filelist.maf file to add conditionally sync_posix.cpp file
2. Parts that are platform-dependent and implement POSIX version of sync are moved to sync_posix.cpp file.
3. No other changes should be observed, especially changes in the moved parts.

[ethouris]

Extra fix: Removed inline from the declaration of create_thread because it was causing a build break.