_free_r() function

Replace standard __swrite function for GDB

This is the replacement for newlib's _REENT_INIT_PTR and __sinit. The problem with __sinit is that it allocates three FILE structures (stdin, stdout, stderr). Having individual standard streams for each task is a bit too much on a small embedded system. So we point streams to the streams of the global struct _reent, which are initialized in startup code.

These functions are to be used in newlib syscall table. They will be called by newlib when it needs to use any of the syscalls.