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

fusl/src/signal/sigaltstack.c

Index: fusl/src/signal/sigaltstack.c
diff --git a/fusl/src/signal/sigaltstack.c b/fusl/src/signal/sigaltstack.c
new file mode 100644
index 0000000000000000000000000000000000000000..62cb81adfe16ea1a112448f1654d55a859718374
--- /dev/null
+++ b/fusl/src/signal/sigaltstack.c
@@ -0,0 +1,18 @@
+#include <signal.h>
+#include <errno.h>
+#include "syscall.h"
+
+int sigaltstack(const stack_t *restrict ss, stack_t *restrict old)
+{
+	if (ss) {
+		if (ss->ss_size < MINSIGSTKSZ) {
+			errno = ENOMEM;
+			return -1;
+		}
+		if (ss->ss_flags & ~SS_DISABLE) {
+			errno = EINVAL;
+			return -1;
+		}
+	}
+	return syscall(SYS_sigaltstack, ss, old);
+}