GEC-50 Put #ifdefs around entire Reduction function

Maik Nijhuisrequested to merge
gec-50-guard-entire-reduction-function into master

Instead of adding empty functions, IDG shouldn't contain PPC and AVX implemenations at all if the architecture doesn't support them.

Moving the #ifdef guards around easily solves the issue and avoids all compiler warnings about unused variables in the empty functions.

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