sys/newlib: drop workaround for stdio

This drops a workaround that initialized newlib's reentrancy structure
on boot to reduce the chances of crashes when using the non-thread-safe
(unless reentrancy hooks are provided) stdio implementation of newlib.

Now that the newlib stdio implementation is only ever used if it is
thread-safe, we no longer need a workaround that reduces the chance
of crashes on concurrent use of stdio.

Author: maribu

sys/newlib_syscalls_default/syscalls.c

@@ -144,23 +144,7 @@ static const struct heap heaps[NUM_HEAPS] = {
  */
 void _init(void)
 {
-    /* Definition copied from newlib/libc/stdio/local.h */
-    extern void __sinit (struct _reent *);
-
-    /* When running multiple threads: Initialize reentrant structure before the
-     * scheduler starts. This normally happens upon the first stdio function
-     * called. However, if no boot message happens this can result in two
-     * concurrent "first calls" to stdio in data corruption, if no locking is
-     * used. Except for ESP (which is using its own syscalls.c anyway), this
-     * currently is the case in RIOT. */
-    if (MAXTHREADS > 1) {
-        /* Also, make an exception for riotboot, which does not use stdio
-         * at all. This would pull in stdio and increase .text size
-         * significantly there */
-        if (!IS_USED(MODULE_RIOTBOOT)) {
-            __sinit(_REENT);
-        }
-    }
+    /* nothing to do here */
 }
 
 /**