[flang][nfc] Move `external-hello-world` to flang/examples

As `external-hello-world` is not really a test, I am moving it from
`flang/unittest/Runtime` to `flang/examples` (it makes a lot of sense as
an example). I've not modified the source code (apart from adjusting the
include paths).

Differential Revision: https://reviews.llvm.org/D104320