Currently, this is a rather useless patch - even though it helps to reduce
the number of local patches I have. Due to the printed sorry, adding a
testcase with -fdump-tree-* is also not possible, yet.

For reverse offload, the plan is to call
   GOMP_target_ext
inside the on the device, passing
   'device(omp_initial_device)'
alias device(GOMP_DEVICE_HOST_FALLBACK) to the
target device's libgomp.

The pointer to the generated target-region function is then
passed as argument. However, that only works if that function
is not nullified ...

The reason that nullifying was added is:
   https://gcc.gnu.org/PR100573
   https://gcc.gnu.org/r12-1066-g95d67762171f83277a5700b270c0d1e2756f83f4
   https://gcc.gnu.org/pipermail/gcc-patches/2021-May/571285.html


Note: Instead of just checking for GOMP_DEVICE_HOST_FALLBACK,
more effort could be done, e.g. by setting some attribute on the
generated function and then check for check for it. Example:
'omp target device_ancestor' + using lookup_attribute).

That's what's done in the second variant.

OK for mainline (which variant)? Or do you prefer to wait for
a more complete patch?

Tobias

PS: Reverse offload - still to do:
- 'requires' patch
- Generate two variants of the target-region function:
   an empty version on the device (just to have a pointer address
   in the offload_func table) and the full version (on the host only)
Those together are sufficient for a omp_get_num_device() == 0 version
(implied by 'required reverse_offload' not being fulfilled by any device).
For a more useful implementation, more work inside libgomp is required.
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