[flang] Handle -S assemble only flag in flang-to-external-fc (#78979)

Flang was recently updated on Compiler Explorer and by default it's in
assemble only mode, you have to enable linking and executing.

This means that the default output for flang-to-external-fc is nothing,
as it doesn't know what `-S` means. You'd have to know to enable the
link to binary option to see any output.

Handle `-S` so that users of Compiler Explorer don't have to wonder why
the "compiler" is broken.

flang/tools/f18/flang-to-external-fc.in

@@ -108,6 +108,10 @@ parse_args()
           COMPILE_ONLY="True"
         fi
 
+        if [[ $1 == "-S" ]]; then
+          COMPILE_ONLY="True"
+        fi
+
         if [[ $1 == "-E" ]]; then
           PREPROCESS_ONLY="True"
         fi