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

fusl/src/thread/tss_create.c

Index: fusl/src/thread/tss_create.c
diff --git a/fusl/src/thread/tss_create.c b/fusl/src/thread/tss_create.c
new file mode 100644
index 0000000000000000000000000000000000000000..251d22b9abe524e5e4ac4f9dd9f6a99850fc68e3
--- /dev/null
+++ b/fusl/src/thread/tss_create.c
@@ -0,0 +1,11 @@
+#include <threads.h>
+
+int __pthread_key_create(tss_t *, void (*)(void *));
+
+int tss_create(tss_t *tss, tss_dtor_t dtor)
+{
+	/* Different error returns are possible. C glues them together into
+	 * just failure notification. Can't be optimized to a tail call,
+	 * unless thrd_error equals EAGAIN. */
+	return __pthread_key_create(tss, dtor) ? thrd_error : thrd_success;
+}