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

fusl/src/stdio/stdin.c

Index: fusl/src/stdio/stdin.c
diff --git a/fusl/src/stdio/stdin.c b/fusl/src/stdio/stdin.c
new file mode 100644
index 0000000000000000000000000000000000000000..171ff22a9842dc74db9ccf1be15df17cfc2af2c3
--- /dev/null
+++ b/fusl/src/stdio/stdin.c
@@ -0,0 +1,15 @@
+#include "stdio_impl.h"
+
+static unsigned char buf[BUFSIZ+UNGET];
+static FILE f = {
+	.buf = buf+UNGET,
+	.buf_size = sizeof buf-UNGET,
+	.fd = 0,
+	.flags = F_PERM | F_NOWR,
+	.read = __stdio_read,
+	.seek = __stdio_seek,
+	.close = __stdio_close,
+	.lock = -1,
+};
+FILE *const stdin = &f;
+FILE *volatile __stdin_used = &f;