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

fusl/src/thread/pthread_attr_setstack.c

Index: fusl/src/thread/pthread_attr_setstack.c
diff --git a/fusl/src/thread/pthread_attr_setstack.c b/fusl/src/thread/pthread_attr_setstack.c
new file mode 100644
index 0000000000000000000000000000000000000000..61707a318993339fa48f338de39c5a4ace0549a2
--- /dev/null
+++ b/fusl/src/thread/pthread_attr_setstack.c
@@ -0,0 +1,9 @@
+#include "pthread_impl.h"
+
+int pthread_attr_setstack(pthread_attr_t *a, void *addr, size_t size)
+{
+	if (size-PTHREAD_STACK_MIN > SIZE_MAX/4) return EINVAL;
+	a->_a_stackaddr = (size_t)addr + size;
+	a->_a_stacksize = size - DEFAULT_STACK_SIZE;
+	return 0;
+}