[Bug c++/93821] Define __cplusplus to 202002L in C++20

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93821

--- Comment #10 from Marco Trevisan <mail at 3v1n0 dot net> ---
I see the point, but then I also think that gcc-10 should not support
`--std=c++20` option but rather only the `--std=c++2a` one.