[llvm] [libc] Refactor scanf reader to match printf (PR #66023)

[Joseph Huber via llvm-commits]

================
@@ -30,7 +30,7 @@ General Flags:
 These compile-time flags will change the behavior of LLVM-libc's printf when it
 is compiled. Combinations of flags that are incompatible will be marked.
 
-LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+LIBC_COPT_STDIO_USE_SYSTEM_FILE
 --------------------------------
----------------
jhuber6 wrote:

```suggestion
-------------------------------
```
One dash too many.

https://github.com/llvm/llvm-project/pull/66023