(PUP-391) Use FFI.errno instead of GetLastError

 - In commit bf6f5c4e67c527d97da2dd3ee692692c8b9a8981, @@GetLastError
   was introduced as a band-aid to ensure the appropriate error code
   was read out of the thread local storage after a Win32 API call was
   made.  At that time, not all of the code had been converted to FFI
   compatible and not all gems had been upgraded to FFI compatible.
   Calls still made using Win32API were not captured by FFI.errno (which
   only calls GetLastError *after* making an FFI defined Win32 API call)
   resulting in a number of test fails.  The solution at the time was
   to manually call GetLastError instead to retrieve the value set by
   SetLastError.
 - Now that all Win32 API calls are made through FFI, FFI will
   automatically capture API results by calling GetLastError, and we
   can remove the unnecessary binding to GetLastError here.