This patch adds support to handle reverse offload to libgomp's plugin-gcn.c and
to AMD GCN's libgomp target.c.

In theory, that's all whats needed for GCN – in practice there a known issue with
private stack variables which has to be addressed independently. Once this and
the target.c generic code is committed, omp requires reverse-offload
support can be claimed for the device (→ GOMP_OFFLOAD_get_num_devices).

Note: Contrary to nvptx, the code to handle reverse offload is already enabled
if there is 'omp requires reverse_offload' (+ target functions) in the code;
for nvptx, an actual reverse-offload-target region has to exist in the code.
This probably does not matter that much in practice.

The '#if 1' code block inside plugin-gcn.c has to be replaced by the
target.c/libgomp-plugins.h/libgomp.h/libgomp.map patch that is part
of the patch: https://gcc.gnu.org/pipermail/gcc-patches/2022-September/602321.html
"[Patch] libgomp/nvptx: Prepare for reverse-offload callback handling"

Andrew did suggest a while back to piggyback on the console_output handling,
avoiding another atomic access. - If this is still wanted, I like to have some
guidance regarding how to actually implement it.

Comments, suggestions?
If not, OK for mainline?*

Tobias

*Without the '#if 1' code and once the non-nvptx bits of the other patch
have been approved and committed.


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