Add GNU-specific strerror_r in filemgr_ops_linux.cc

- In GNU-specific strerror_r, the error string may not be copied in
the buffer passed by caller (this happens when the string is immutable
and static), thus we have to use the returned string pointer instead of
'tbuf'.
Merge "Add GNU-specific strerror_r in filemgr_ops_linux.cc"