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

fusl/src/thread/thrd_sleep.c

Index: fusl/src/thread/thrd_sleep.c
diff --git a/fusl/src/thread/thrd_sleep.c b/fusl/src/thread/thrd_sleep.c
new file mode 100644
index 0000000000000000000000000000000000000000..e8dfe400cb701f7ae470b192ec04c495a7bfbb18
--- /dev/null
+++ b/fusl/src/thread/thrd_sleep.c
@@ -0,0 +1,13 @@
+#include <threads.h>
+#include <errno.h>
+#include "syscall.h"
+
+int thrd_sleep(const struct timespec *req, struct timespec *rem)
+{
+	int ret = __syscall(SYS_nanosleep, req, rem);
+	switch (ret) {
+	case 0:      return 0;
+	case -EINTR: return -1; /* value specified by C11 */
+	default:     return -2;
+	}
+}