Add a "fork" of musl as //fusl.

fusl/src/thread/pthread_mutex_consistent.c

Index: fusl/src/thread/pthread_mutex_consistent.c
diff --git a/fusl/src/thread/pthread_mutex_consistent.c b/fusl/src/thread/pthread_mutex_consistent.c
new file mode 100644
index 0000000000000000000000000000000000000000..96b83b52851b202aed9f817447129792833d1da1
--- /dev/null
+++ b/fusl/src/thread/pthread_mutex_consistent.c
@@ -0,0 +1,10 @@
+#include "pthread_impl.h"
+
+int pthread_mutex_consistent(pthread_mutex_t *m)
+{
+	if (!(m->_m_type & 8)) return EINVAL;
+	if ((m->_m_lock & 0x7fffffff) != __pthread_self()->tid)
+		return EPERM;
+	m->_m_type &= ~8U;
+	return 0;
+}