Make GCC backend explicitly fail on tail call attempts

Author: semtexzv

Diff for: compiler/rustc_codegen_gcc/src/builder.rs

@@ -1687,8 +1687,8 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
     }
 
     fn set_tail_call(&mut self, _call_inst: RValue<'gcc>) {
-        // TODO: Implement proper tail call optimization for GCC backend
-        // For now, this is a no-op to make compilation work
+        // Explicitly fail when this method is called
+        bug!("Guaranteed tail calls with the 'become' keyword are not implemented in the GCC backend");
     }
 
     fn set_span(&mut self, _span: Span) {}