sys/posix/pthread: newlib compatibility

When using a toolchain with built-in POSIX thread support, static C++ constructors use a static mutex variable which is initialized with `pthread_once` when first used. However, since RIOT's `pthread_once_t` type is different from that in newlib's `pthread`, which is assumed by GCC, RIOT crashes as soon as static constructors are used.
Changing the `pthread_once_t` type to be compatible with newlib's `pthread_once_t` type solves the problem and allows the RIOT `pthread` modules to be used even with toolchains with built-in POSIX thread support.

sys/posix/pthread/include/pthread_once.h

@@ -24,14 +24,17 @@ extern "C" {
 /**
  * @brief           Datatype to supply to pthread_once().
  */
-typedef volatile int pthread_once_t;
+typedef struct {
+  int   is_initialized;  /* initialized */
+  int   init_executed;   /* init function executed */
+} pthread_once_t;
 
 /**
  * @def             PTHREAD_ONCE_INIT
  * @brief           Initialization for pthread_once_t.
  * @details         A zeroed out pthread_once_t is initialized.
  */
-#define PTHREAD_ONCE_INIT 0
+#define PTHREAD_ONCE_INIT  { 1, 0 }
 
 /**
  * @brief           Helper function that ensures that `init_routine` is called at once.

sys/posix/pthread/pthread_once.c

@@ -22,11 +22,11 @@
 
 int pthread_once(pthread_once_t *once_control, void (*init_routine)(void))
 {
-    if (*once_control == PTHREAD_ONCE_INIT) {
+    if (!once_control->init_executed) {
         init_routine();
     }
 
-    *once_control = PTHREAD_ONCE_INIT + 1;
+    once_control->init_executed = 1;
 
     return 0;
 }