Set host_arch to ia32 on machines with a 32bit userland but a 64bit kernel.

I don't know if there are any v8 bots with that configuration, but it seems
like a good idea to have v8 be consistent with chromium and nacl here, so that
this works fine if such a bot is ever set up.

This is similar to https://codereview.chromium.org/342493002/ , but with a
detect_v8_host_arch.py script that handles the additional machine types that
standalone.gypi used to handle.

Originally reviewed at https://codereview.chromium.org/346643002/

BUG=368384
LOG=Y
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/349333006

Patch from Nico Weber <thakis@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21956 ce2b1a6d-e550-0410-aec6-3dcde31c8c00