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

fusl/src/stdio/stderr.c

Index: fusl/src/stdio/stderr.c
diff --git a/fusl/src/stdio/stderr.c b/fusl/src/stdio/stderr.c
new file mode 100644
index 0000000000000000000000000000000000000000..229c8651dd5bd14e31ada5bb542f0c62f19a55bf
--- /dev/null
+++ b/fusl/src/stdio/stderr.c
@@ -0,0 +1,16 @@
+#include "stdio_impl.h"
+
+static unsigned char buf[UNGET];
+static FILE f = {
+	.buf = buf+UNGET,
+	.buf_size = 0,
+	.fd = 2,
+	.flags = F_PERM | F_NORD,
+	.lbf = -1,
+	.write = __stdio_write,
+	.seek = __stdio_seek,
+	.close = __stdio_close,
+	.lock = -1,
+};
+FILE *const stderr = &f;
+FILE *volatile __stderr_used = &f;