Enable dispatching against the xsimd::generic architecture

[amyspark]

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Vc has a "Scalar" architecture that allows the library to be used without targeting any particular ISA, hence doing all computations on individual values.

xsimd does have something similar (xsimd::generic) that is the fallback for all architecture types. Would it be possible to complete its definition, if even with a dummy register type (a single T value), so as to make xsimd usable on potentially unknown instruction sets?