Restore _lock initialization in non-single threaded mode

When __SINGLE_THREAD__ is not defined, stdin, stdout and stderr needs
to have their _lock instance initialized. The __sfp() method is not
invoked for the 3 mentioned fds thus, the std() method needs to handle
the initialization of the lock.

This is more or less a revert of 382550072b

Contributed by STMicroelectronics

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>

newlib/libc/stdio/findfp.c

@@ -88,6 +88,10 @@ std (FILE *ptr,
 #else /* _STDIO_CLOSE_STD_STREAMS */
   ptr->_close = NULL;
 #endif /* _STDIO_CLOSE_STD_STREAMS */
+#ifndef __SINGLE_THREAD__
+  if (ptr == &__sf[0] || ptr == &__sf[1] || ptr == &__sf[2])
+    __lock_init_recursive (ptr->_lock);
+#endif
 #ifdef __SCLE
   if (__stextmode (ptr->_file))
     ptr->_flags |= __SCLE;