propose adding host v. target architecture mismatch to problem list

I spent an afternoon scratching my head on this one before I ran
into #1728.

Since the error message welcomes PRs...

Author: bunnie

src/ir/context.rs

@@ -575,6 +575,7 @@ impl BindgenContext {
 - Unrecognized flags
 - Invalid flag arguments
 - File I/O errors
+- Host vs. target architecture mismatch
 If you encounter an error missing from this list, please file an issue or a PR!")
         };