This patch does three things:

(a) It removes a 'sorry' for 'device(ancestor:1)' and passes
     GOMP_DEVICE_HOST_FALLBACK as device number.

This is sufficient for full "reverse" offload support with ENABLE_OFFLOADING
being false - and -foffload=disable. And for simple hello-world cases.


On the libgomp side, the 'requires reverse_offload' currently implies that
the initial device is the only device. While that's all fine, this change
is insufficient if offloading devices are enabled during compilation as:


(b.1) The offload-device lto1 should not see the content of the ancestor:1 target
region and all the calls it does. If it does, there will be link errors for
functions not available and it also would pointlessly increase the code size.

Thus, the second part is to create an empty function for devices and a full
version for the host.

The general idea is: The device version can be used as lookup pointer in the
offload_funcs table; thus, we both need a function on the device and a call to
GOMP_target_ext.

It turned out to be quite difficult as late in the processing changing a
FUNCTION_DECL is not that easy – nor removing it after all analysis has been
done. I hope the current version is not too hackish – and maybe someone has
an idea how to best not to assembly the 'nonhost' version on the host.
(Not critical as it is small (having an empty body) - but still it would be
nicer not to write it to .s file.)


(b.2) The omp-offload.cc assert showed that cloning and inlining happened
for the included libgomp example. While inlining should be okay (of
'subroutine m2_tg_fn' (and for C/C++ 'tg_fn')) - cloning will break
the offload_func table lookup - and, hence, had to be excluded → "noclone".
I think it could also affect non-anchestor:1 code - but did not try to
create an example.


(c) Prepare for actual reverse offloading
While (b) already does some prep work for real offloading, at least one more
step is needed: In order to allow that the function pointer can be used for
offload_func table lookup, it has to be passed to libgomp.

Currently, the 'fn' argument is nullified in on-device calls to GOMP_target_ext.
The third part of this patch nullifies it now only for non-reverse offloads.

OK for mainline?

  * * *

Next steps: Implement reverse offloading for devices. In theory, this only
requires libgomp work, but let's see what else will be required.

Tobias
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