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

fusl/src/thread/thrd_create.c

Index: fusl/src/thread/thrd_create.c
diff --git a/fusl/src/thread/thrd_create.c b/fusl/src/thread/thrd_create.c
new file mode 100644
index 0000000000000000000000000000000000000000..e03366952657117198219b49999d2b3985500fd3
--- /dev/null
+++ b/fusl/src/thread/thrd_create.c
@@ -0,0 +1,14 @@
+#include "pthread_impl.h"
+#include <threads.h>
+
+int __pthread_create(pthread_t *restrict, const pthread_attr_t *restrict, void *(*)(void *), void *restrict);
+
+int thrd_create(thrd_t *thr, thrd_start_t func, void *arg)
+{
+	int ret = __pthread_create(thr, __ATTRP_C11_THREAD, (void *(*)(void *))func, arg);
+	switch (ret) {
+	case 0:      return thrd_success;
+	case EAGAIN: return thrd_nomem;
+	default:     return thrd_error;
+	}
+}