build: teach nodejs check about machine arch

Otherwise (on my PowerPC NAS) it just downloads and unpacks the x86 archive for nodejs, eventually failing with an incorrect message that I have an old nodejs (since it can't get the version correctly).

Checked that this didn't break anything on x86. Btw, there's a package called 'node' in Ubuntu repos that also has a /usr/sbin/node which is picked up by the `which node`. Not much to be done here.

Btw, I thought of adding compilation from source for the architectures for which there are no binaries to 'do', but decided that nodejs is in the Debian repo, so the users are better off installing it from there. (Btw, it's in wheezy-backports, which is sadly not compiled for powerpc, but this is too far a tangent).