[flang] Add target/triple in a test

A test added in https://reviews.llvm.org/D127207 is missing
target/triple. This has caused the PowerPC buildbot to start failing:
  * https://lab.llvm.org/buildbot/#/builders/21/builds/42860
(on PowerPC `; CHECK: ret` should be replaced with `; CHECK: `blr`).

Sending this without a review as the fix is rather straightforward. Note
that I've decided to add triple/target instead of e.g. removing:
`; CHECK: ret`. That's for consistency with other tests that generate
assembly. We could change that if that's what folks prefer.