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

fusl/src/signal/sigsetjmp_tail.c

Index: fusl/src/signal/sigsetjmp_tail.c
diff --git a/fusl/src/signal/sigsetjmp_tail.c b/fusl/src/signal/sigsetjmp_tail.c
new file mode 100644
index 0000000000000000000000000000000000000000..78762aa2bf8d141737a4ba52ed34fac94308aa52
--- /dev/null
+++ b/fusl/src/signal/sigsetjmp_tail.c
@@ -0,0 +1,11 @@
+#include <setjmp.h>
+#include <signal.h>
+#include "syscall.h"
+
+__attribute__((__visibility__("hidden")))
+int __sigsetjmp_tail(sigjmp_buf jb, int ret)
+{
+	void *p = jb->__ss;
+	__syscall(SYS_rt_sigprocmask, SIG_SETMASK, ret?p:0, ret?0:p, _NSIG/8);
+	return ret;
+}