Threads abstraction (#3423)

* Threads and synchronization abstractions for paparazzi.
* Test module for the threads abstraction.
* Replace old mutexes by the new ones.

---------

Co-authored-by: Fabien-B <Fabien-B@github.com>

tests/modules/test_arch/modules/core/threads_arch.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#define PPRZ_NORMAL_PRIO 128
+
+#define THREADS_ATTRIBUTES 
+
+struct pprzMutex {
+};
+
+struct pprzBSem {
+};
+
+
+typedef struct{} pprz_thread_t;