[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.