MDEV-32777 DDL operations very slow under macOS

macOS is a development platform for our product but calls to
fcntl(F_FULLFSYNC) are too slow for developer needs.  macOS offers
at least three different disk flush calls, making tradeoffs between
reliability and performance (see analysis notes on this MDEV).
On macOS only and as a compromise, we will back out the change to
call fcntl(F_FULLFSYNC), sacrificing a full flush in favor of more
performant writes.

mysys/my_sync.c

@@ -70,17 +70,7 @@ int my_sync(File fd, myf my_flags)
 
  do
  {
-#if defined(F_FULLFSYNC)
- /*
- In Mac OS X >= 10.3 this call is safer than fsync() (it forces the
- disk's cache and guarantees ordered writes).
- */
- if (!(res= fcntl(fd, F_FULLFSYNC, 0)))
- break; /* ok */
- /* Some file systems don't support F_FULLFSYNC and fail above: */
- DBUG_PRINT("info",("fcntl(F_FULLFSYNC) failed, falling back"));
-#endif
-#if defined(HAVE_FDATASYNC) && HAVE_DECL_FDATASYNC
+#if defined(HAVE_FDATASYNC) && defined(HAVE_DECL_FDATASYNC)
  res= fdatasync(fd);
 #elif defined(HAVE_FSYNC)
  res= fsync(fd);