Use re-entrant locks on stdio

libc/intrin/gettid.greg.c

@@ -27,6 +27,12 @@ __msabi extern typeof(GetCurrentThreadId) *const __imp_GetCurrentThreadId;
 
 /**
  * Returns current thread id.
+ *
+ * On Linux, and Linux only, this is guaranteed to be equal to getpid()
+ * if this is the main thread. On NetBSD, gettid() for the main thread
+ * is always 1.
+ *
+ * @return thread id greater than zero or -1 w/ errno
  * @asyncsignalsafe
  */
 privileged int gettid(void) {

libc/intrin/spinlock.h

@@ -64,7 +64,7 @@
   do {                                               \
     autotype(lock) __lock = (lock);                  \
     typeof(*__lock) __x;                             \
-    int __tries = 0;                                 \
+    unsigned __tries = 0;                            \
     for (;;) {                                       \
       __atomic_load(__lock, &__x, __ATOMIC_RELAXED); \
       if (!__x && !_trylock_inline(__lock)) {        \