Don't enable WPO on Win64 and require Server 2003 / x64 for win64

WPO was only enabled to work around an issue with the Isolate's static
initializer which no longer exists.

Requiring XP on Win64 doesn't make sense.

BUG=421363
R=machenbach@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/637303002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24457 ce2b1a6d-e550-0410-aec6-3dcde31c8c00