Add comment why thiserror is not used

cranelift/codegen/src/isa/unwind/systemv.rs

@@ -21,6 +21,8 @@ pub enum RegisterMappingError {
     UnsupportedRegisterBank(&'static str),
 }
 
+// This is manually implementing Error and Display instead of using thiserror to reduce the amount
+// of dependencies used by Cranelift.
 impl std::error::Error for RegisterMappingError {}
 
 impl std::fmt::Display for RegisterMappingError {