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

fusl/src/thread/pthread_setschedprio.c

Index: fusl/src/thread/pthread_setschedprio.c
diff --git a/fusl/src/thread/pthread_setschedprio.c b/fusl/src/thread/pthread_setschedprio.c
new file mode 100644
index 0000000000000000000000000000000000000000..e0bdc03b8405c2f8938bbf557b933e375e9dce81
--- /dev/null
+++ b/fusl/src/thread/pthread_setschedprio.c
@@ -0,0 +1,10 @@
+#include "pthread_impl.h"
+
+int pthread_setschedprio(pthread_t t, int prio)
+{
+	int r;
+	__lock(t->killlock);
+	r = t->dead ? ESRCH : -__syscall(SYS_sched_setparam, t->tid, &prio);
+	__unlock(t->killlock);
+	return r;
+}