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'`

pkgs/development/compilers/nim/default.nix

···
       "mips64"
     else if isMsp430 then
       "msp430"
-    else if isPowerPC && is32bit then
+    else if isPower && is32bit then
       "powerpc"
-    else if isPowerPC && is64bit then
+    else if isPower && is64bit then
       "powerpc64"
     else if isRiscV && is64bit then
       "riscv64"