SERVER-14835 - fix fassert 16143 on PPC64 LE systems in LogFile::synchronousAppend()

new version, using ioctl(_fd, BLKBSZGET) and fixed predefined alignment for PPC64.
Can we make g_minOSPageSizeBytes the default for _blkSize and move the ioctl call into the the  O_DIRECT #ifdef...#endif  block around lines 186-195