Merge pull request #5249 from gebart/pr/newlib-readwrite-prototypes

sys/newlib: Update _read_r and _write_r to match newlib prototypes in reent.h

sys/newlib/syscalls.c

@@ -186,7 +186,7 @@ int _open_r(struct _reent *r, const char *name, int flags, int mode)
  *
  * @return      TODO
  */
-int _read_r(struct _reent *r, int fd, void *buffer, unsigned int count)
+_ssize_t _read_r(struct _reent *r, int fd, void *buffer, size_t count)
 {
     (void)r;
     (void)fd;
@@ -208,7 +208,7 @@ int _read_r(struct _reent *r, int fd, void *buffer, unsigned int count)
  *
  * @return      TODO
  */
-int _write_r(struct _reent *r, int fd, const void *data, unsigned int count)
+_ssize_t _write_r(struct _reent *r, int fd, const void *data, size_t count)
 {
     (void) r;
     (void) fd;