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

fusl/src/thread/pthread_mutex_init.c

Index: fusl/src/thread/pthread_mutex_init.c
diff --git a/fusl/src/thread/pthread_mutex_init.c b/fusl/src/thread/pthread_mutex_init.c
new file mode 100644
index 0000000000000000000000000000000000000000..acf45a74687bcf3d8dfb6ac7d4e9107d922b4fe2
--- /dev/null
+++ b/fusl/src/thread/pthread_mutex_init.c
@@ -0,0 +1,8 @@
+#include "pthread_impl.h"
+
+int pthread_mutex_init(pthread_mutex_t *restrict m, const pthread_mutexattr_t *restrict a)
+{
+	*m = (pthread_mutex_t){0};
+	if (a) m->_m_type = a->__attr;
+	return 0;
+}