Allowing -std=c2y flag, as an alias to c23

[aalmkainzi]

Simple change. Ideally the -std=c2y flag should enable c2y extensions that slimcc implements, and -std=c23 should disable them. But this is fine.

[fuhsnn]

slimcc's target mode is roughly "what GCC/Clang allow under -std=cXX -Wno-pedantic", and all C2Y stuff currently implemented here are enabled by GCC under even -std=c89.
Yeah, doubled checked https://godbolt.org/z/To6fvf7bd.

The rule (which I believe GCC/Clang maintainers generally agree) is if the same syntax yield different result then it should be -std gated, but if something simply "won't appear" in older standard code, then it is enabled as extension unless -pedantic.

[aalmkainzi]

I think the gcc example worked because of the countof header (probably expands to some __builtin if not C2y).

I tried using the _Countof keyword directly, and it didn't work in C89: https://godbolt.org/z/P7an9djao

[fuhsnn]

The error is for the // comment (C99) https://godbolt.org/z/9xa8jddhq

_Countof itself is the builtin: https://github.com/gcc-mirror/gcc/blob/9f99ee9d103dd70c18637271bb7f9b3efc926959/gcc/ginclude/stdcountof.h#L29
Underscore+uppercase is reserved like double underscore.