Introduce _REENT_GLOBAL_STDIO_STREAMS

Introduce _REENT_GLOBAL_STDIO_STREAMS

In Newlib, the stdio streams are defined to thread-specific pointers
_reent::_stdin, _reent::_stdout and _reent::_stderr.  In case
_REENT_SMALL is not defined, then these pointers are initialized via
_REENT_INIT_PTR() or _REENT_INIT_PTR_ZEROED() to thread-specific FILE
objects provided via _reent::__sf[3].  There are two problems with this
(at least in case of RTEMS).

(1) The thread-specific FILE objects are closed by _reclaim_reent().
This leads to problems with language run-time libraries that provide
wrappers to the C/POSIX stdio streams (e.g.  C++ and Ada), since they
use the thread-specific FILE objects of the initialization thread.  In
case the initialization thread is deleted, then they use freed memory.

(2) Since thread-specific FILE objects are used with a common output
device via file descriptors 0, 1 and 2, the locking at FILE object level
cannot ensure atomicity of the output, e.g. a call to printf().

Introduce a new Newlib configuration option _REENT_GLOBAL_STDIO_STREAMS
to enable the use of global stdio FILE objects.

As a side-effect this reduces the size of struct _reent by more than
50%.

The _REENT_GLOBAL_STDIO_STREAMS should not be used without
_STDIO_CLOSE_PER_REENT_STD_STREAMS.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>