GCN, nvptx libstdc++: Force use of '__atomic' builtins: revert 'atomicity_dir=cpu...

GCN, nvptx libstdc++: Force use of '__atomic' builtins: revert 'atomicity_dir=cpu/generic/atomicity_builtins' hard-coding [PR119645]"

Thanks to commit 86627faec10da53d7532805019e5296fcf15ac09
"libstdc++: Rewrite atomic builtin checks [PR70560]", for both GCN, nvptx
we now get:

    +configure:16060: checking for atomic builtins for _Atomic_word
    +[...]
    +configure:16073: result: yes

..., and thus may revert the 'atomicity_dir=cpu/generic/atomicity_builtins'
hard-coding added in commit 059b5509c14904b55c37f659170240ae0d2c1c8e
"GCN, nvptx libstdc++: Force use of '__atomic' builtins [PR119645]".

	PR target/119645
	libstdc++-v3/
	* configure.host [GCN, nvptx] (atomicity_dir): Don't set.