fix node to build with target_arch=x64 on linux

atom-shell on linux is incorrectly reporting `ia32` for `process.arch`.

This is happening because `-Dtarget_arch=ia32` is passed to ninja on linux inside `script/update.py` which leads to '-DARCH="ia32"' being set in the compile flags. I see that the current intention is to target 64 bit builds on linux (37275c64cd07) and the binaries are in fact compiled as 64 bit despite this bug.  I guess ninja is somehow smartly ignoring the incorrect setting of the `-m32` flags at https://github.com/atom/node/blob/6d772c3cda0bb8270857397e128ea1e36c361125/common.gypi#L175-L178.

Until this is fixed it breaks usage of any node-pre-gyp packaged node addons because node-pre-gyp depends on process.arch being correct in order to require the right binary arch.
This sets `target_arch` to `x64` on 32bit Linux, which would result in a new bug, on Linux we need to set `target_arch` according to current machine's arch.