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

fusl/src/signal/sigfillset.c

Index: fusl/src/signal/sigfillset.c
diff --git a/fusl/src/signal/sigfillset.c b/fusl/src/signal/sigfillset.c
new file mode 100644
index 0000000000000000000000000000000000000000..16e7b4f5bc420516ad70ef636524fe4084dfc730
--- /dev/null
+++ b/fusl/src/signal/sigfillset.c
@@ -0,0 +1,18 @@
+#include <signal.h>
+#include <limits.h>
+
+int sigfillset(sigset_t *set)
+{
+#if ULONG_MAX == 0xffffffff
+	set->__bits[0] = 0x7ffffffful;
+	set->__bits[1] = 0xfffffffcul;
+	if (_NSIG > 65) {
+		set->__bits[2] = 0xfffffffful;
+		set->__bits[3] = 0xfffffffful;
+	}
+#else
+	set->__bits[0] = 0xfffffffc7ffffffful;
+	if (_NSIG > 65) set->__bits[1] = 0xfffffffffffffffful;
+#endif
+	return 0;
+}