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

fusl/src/thread/pthread_setspecific.c

Index: fusl/src/thread/pthread_setspecific.c
diff --git a/fusl/src/thread/pthread_setspecific.c b/fusl/src/thread/pthread_setspecific.c
new file mode 100644
index 0000000000000000000000000000000000000000..55e46a899378f4f896e570ad8edbacbe569cbcf8
--- /dev/null
+++ b/fusl/src/thread/pthread_setspecific.c
@@ -0,0 +1,12 @@
+#include "pthread_impl.h"
+
+int pthread_setspecific(pthread_key_t k, const void *x)
+{
+	struct pthread *self = __pthread_self();
+	/* Avoid unnecessary COW */
+	if (self->tsd[k] != x) {
+		self->tsd[k] = (void *)x;
+		self->tsd_used = 1;
+	}
+	return 0;
+}