lib/systems/inspect.nix: add isPower64
lib/systems/inspect.nix: add isPower64

This commit adds an `isPower64` predicate to the two existing
predicates for this architecture (`isPower` and `isPowerPC`).

Note that `isPowerPC` matches only 32-bit machines, whereas `isPower`
matches both 64-bit and 32-bit machines.  Prior to this commit there
was no single `isXXX` predicate for `powerpc64le`.

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