Optionally use thread-local objects for struct _reent members

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

Hello,

Newlib currently uses a huge object of type struct _reent to store 
thread-specific data. This object is returned by __getreent() if the 
__DYNAMIC_REENT__ Newlib configuration option is defined.

The reentrancy structure contains errno and also the standard input, 
output, and error file streams. This means that if an application only 
uses errno it has a dependency on the file stream support even if it 
does not use it. This is an issue for lower end targets and applications 
which need to qualify the software according to safety standards (for 
example ECSS-E-ST-40C, ECSS-Q-ST-80C, IEC 61508,
ISO 26262, DO-178, DO-330, DO-333).

One approach to disentangle the dependencies introduced by struct _reent 
is to get rid of this structure and replace the individual members of 
the structure with thread-local objects. For example, instead of

struct _reent {
   int _errno;
   __FILE *_stdin;
   __FILE *_stdout;
   __FILE *_stderr;
};

use

_Thread_local int _errno;
_Thread_local __FILE *_stdin;
_Thread_local __FILE *_stdout;
_Thread_local __FILE *_stderr;

if a new configuration option is defined (for example _REENT_THREAD_LOCAL).

Newlib already has access macros for some struct _reent members, for 
example:

#define _REENT_SIGNGAM(ptr)     ((ptr)->_new._reent._gamma_signgam)
#define _REENT_RAND_NEXT(ptr)   ((ptr)->_new._reent._rand_next)
#define _REENT_RAND48_SEED(ptr) ((ptr)->_new._reent._r48._seed)
#define _REENT_RAND48_MULT(ptr) ((ptr)->_new._reent._r48._mult)
#define _REENT_RAND48_ADD(ptr)  ((ptr)->_new._reent._r48._add)

The member access macros are incomplete. We would have to add macros for 
all the struct _reent members. Would this be an approach acceptable for 
Newlib integration?

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