Explain why we can't use the compiler-rt functions or write them in C

Author: Amanieu

text/0000-int128.md

@@ -60,7 +60,7 @@ fn __udivti3(a: u128, b: u128) -> u128;
 fn __umodti3(a: u128, b: u128) -> u128;
 ```
 
-Implementations of these functions will be written in Rust and will be included in libcore.
+Implementations of these functions will be written in Rust and will be included in libcore. Note that it is not possible to write these functions in C or use the existing implementations in `compiler-rt` since the `__int128` type is not available in C on 32-bit platforms.
 
 ## Modifications to libcore