xt-declare-target-ctors: Update for GCC

[tschwinge]

It's not clear from the revision log what compiler the xt-declare-target-ctors test has been written against, in particular, its
'xt-declare-target-ctors/expected' file. But it must've been a long time ago.

Updating this for current GCC:

1. Like on the host, also for offloading: destructors run in reverse order of constructors.
2. Offloading is initialized (and with that, offloading constructors are run) upon encountering the first offloading construct (here: '#pragma omp target'), and offloading is shut down (and with that, offloading destructors are run) at the end of 'main', before destructing host objects.

[tschwinge]

Before merging, we should check what other current compilers are doing.