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.
@zcbenz something like this will work, ya?

    if sys.platform.startswith('linux') and sys.maxsize > 2**32:
      arch = 'x64'

@springmeyer want to update PR with this or so?