[libc] Fix forward octal prefix (#73526)

To fix failing build bots after :
https://github.com/llvm/llvm-project/pull/73372

```
/home/libc-buildbot/libc-aarch64-ubuntu/libc-aarch64-ubuntu-fullbuild-dbg/llvm-project/libc/src/__support/macros/properties/float.h:40:71: error: invalid digit '9' in octal constant
#if (defined(LIBC_COMPILER_CLANG_VER) && (LIBC_COMPILER_CLANG_VER >= 0900)) || \
```