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

fusl/src/thread/pthread_rwlockattr_setpshared.c

Index: fusl/src/thread/pthread_rwlockattr_setpshared.c
diff --git a/fusl/src/thread/pthread_rwlockattr_setpshared.c b/fusl/src/thread/pthread_rwlockattr_setpshared.c
new file mode 100644
index 0000000000000000000000000000000000000000..e7061973d6d6b1c00a79cd2c3cd4bb04b0f104d7
--- /dev/null
+++ b/fusl/src/thread/pthread_rwlockattr_setpshared.c
@@ -0,0 +1,8 @@
+#include "pthread_impl.h"
+
+int pthread_rwlockattr_setpshared(pthread_rwlockattr_t *a, int pshared)
+{
+	if (pshared > 1U) return EINVAL;
+	a->__attr[0] = pshared;
+	return 0;
+}