MDEV-8743: where O_CLOEXEC is available, use for innodb buf_dump

As this is the only moderately critical fopened for writing file,
create an alternate path to use open and fdopen for non-glibc platforms
that support O_CLOEXEC (BSDs).

Tested on Linux (by modifing the GLIBC defination) to take this
alternate path:

$ cd /proc/23874
$ more fdinfo/71
pos:    0
flags:  02100001
mnt_id: 24
$ ls -la fd/71
l-wx------. 1 dan dan 64 Mar 14 13:30 fd/71 -> /dev/shm/var_auto_i7rl/mysqld.1/data/ib_buffer_pool.incomplete

storage/innobase/buf/buf0dump.cc

@@ -275,7 +275,20 @@ buf_dump(
 	buf_dump_status(STATUS_INFO, "Dumping buffer pool(s) to %s",
 			full_filename);
 
+#if defined(__GLIBC__) || defined(__WIN__) || O_CLOEXEC == 0
 	f = fopen(tmp_filename, "w" STR_O_CLOEXEC);
+#else
+	{
+		int	fd;
+		fd = open(tmp_filename, O_CREAT | O_TRUNC | O_CLOEXEC | O_WRONLY, 0640);
+		if (fd >= 0) {
+			f = fdopen(fd, "w");
+		}
+		else {
+			f = NULL;
+		}
+	}
+#endif
 	if (f == NULL) {
 		buf_dump_status(STATUS_ERR,
 				"Cannot open '%s' for writing: %s",