make _win.S know if it's 64-bit

I think this is the root of my Windows / Chrome problems.
Even on 32-bit builds, Chrome compiles nacl64.exe in 64-bit mode.

So to make things simple, always put _win.S in the sources,
and no-op it away when assembling for 32-bit.

Change-Id: I19f163491739a6c0cbdedd0ce353f1d2289907ae
Reviewed-on: https://skia-review.googlesource.com/10637
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed