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

fusl/src/linux/sendfile.c

Index: fusl/src/linux/sendfile.c
diff --git a/fusl/src/linux/sendfile.c b/fusl/src/linux/sendfile.c
new file mode 100644
index 0000000000000000000000000000000000000000..d63f4197bb8b8db64470e02b4b157d05058feaef
--- /dev/null
+++ b/fusl/src/linux/sendfile.c
@@ -0,0 +1,10 @@
+#include <sys/sendfile.h>
+#include "syscall.h"
+#include "libc.h"
+
+ssize_t sendfile(int out_fd, int in_fd, off_t *ofs, size_t count)
+{
+	return syscall(SYS_sendfile, out_fd, in_fd, ofs, count);
+}
+
+LFS64(sendfile);