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

fusl/src/thread/pthread_attr_setstacksize.c

Index: fusl/src/thread/pthread_attr_setstacksize.c
diff --git a/fusl/src/thread/pthread_attr_setstacksize.c b/fusl/src/thread/pthread_attr_setstacksize.c
new file mode 100644
index 0000000000000000000000000000000000000000..09d3fda72ee055ba93de2e1534d153ea2f6754b3
--- /dev/null
+++ b/fusl/src/thread/pthread_attr_setstacksize.c
@@ -0,0 +1,9 @@
+#include "pthread_impl.h"
+
+int pthread_attr_setstacksize(pthread_attr_t *a, size_t size)
+{
+	if (size-PTHREAD_STACK_MIN > SIZE_MAX/4) return EINVAL;
+	a->_a_stackaddr = 0;
+	a->_a_stacksize = size - DEFAULT_STACK_SIZE;
+	return 0;
+}