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

fusl/src/linux/signalfd.c

+#include <sys/signalfd.h>
+#include <signal.h>
+#include <errno.h>
+#include <fcntl.h>
+#include "syscall.h"
+
+int signalfd(int fd, const sigset_t *sigs, int flags)
+{
+        int ret = __syscall(SYS_signalfd4, fd, sigs, _NSIG/8, flags);
+#ifdef SYS_signalfd
+        if (ret != -ENOSYS) return __syscall_ret(ret);
+        ret = __syscall(SYS_signalfd, fd, sigs, _NSIG/8);
+        if (ret >= 0) {
+                if (flags & SFD_CLOEXEC)
+                        __syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
+                if (flags & SFD_NONBLOCK)
+                        __syscall(SYS_fcntl, ret, F_SETFL, O_NONBLOCK);
+        }
+#endif
+        return __syscall_ret(ret);
+}