nim: fix power arch predicates
nim: fix power arch predicates

isPowerPC is only true on 32 bit machines;
to check the family we should use isPower here

Otherwise, e.g. `pkgsCross.powernv.buildPackages.nim` fails to eval with
`error: evaluation aborted with the following error message: 'no Nim CPU support known for powerpc64le-unknown-linux-gnu'`

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