[Patch] Add 'default' to -foffload=; document that flag [PR67300]

[Tobias Burnus]

I managed to delete the libgomp part before posting the patch, hence,
reposted.

(The change from -foffload= to -foffload-options= ensures that also
other configured compilers such as GCN are used, an issue that Thomas
found. The original -foffload=nvptx-none=-latomic was added because as
otherwise the GCN part caused build issues for Richard.)

Thus, this patch is like v3, except for the invoke.texi fixes suggested
by Sandra (thanks!) + adding a ChangeLog
and like v4, except the lost libgomp changes has been re-added (+
ChangeLog update).

I hope it now is fine.

Tobias
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