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

fusl/src/thread/tss_set.c

Index: fusl/src/thread/tss_set.c
diff --git a/fusl/src/thread/tss_set.c b/fusl/src/thread/tss_set.c
new file mode 100644
index 0000000000000000000000000000000000000000..70c4fb723cf68b6e37e43477ae8f17d6f6e63b1c
--- /dev/null
+++ b/fusl/src/thread/tss_set.c
@@ -0,0 +1,13 @@
+#include "pthread_impl.h"
+#include <threads.h>
+
+int tss_set(tss_t k, void *x)
+{
+	struct pthread *self = __pthread_self();
+	/* Avoid unnecessary COW */
+	if (self->tsd[k] != x) {
+		self->tsd[k] = x;
+		self->tsd_used = 1;
+	}
+	return thrd_success;
+}