Add a testcase for "omp requires unified_address" as we hadn't one.

The feature itself worked since the beginning (hardware + implementation
wise); that the devices report 'omp requires unified_address' as
supported is newer: for nvptx since r13-3460-g131d18e928a3ea and for GCN
since r14-1584-gf1af7d65ff64fe (a week ago).

The test assumes (→ dg-output) that all offload devices support
unified_address; this implies: if an offloading device is available, it
also remains available after adding the unified-address requirement.
Goal: ensure that we don't end up with only host fallback.

Unified address implies: Pointer size is the same such that no
'is_device_ptr' is required to convert an opaque pointer, it also
permits to do device-pointer pointer arithmetic on the host. This
testcase also assumes that 'int' / 'integer' has the same size on host
and device. (If not: good luck with offloading in general!)

The test also makes the sound assumption that derived-type component
pointers are passed through with the derived-type itself such that the
pointer address remains well defined. In terms of the standard, it would
have an undefined association status.

Committed as Rev. r14-1783-gd5c58ad1ebaff9

Tobias
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