ntdll: Add support for Windows XP 64-bit

[Ken Sharp]

Morning all,

Could someone far smarter than me please take a look at the attached patch?

Windows XP 64-bit runs NT kernel 5.2, whereas 32-bit runs 5.1.  Any
64-bit app expecting XP 64-bit will fail (such as IE8 for NT 5.2).

The patch lets the installer continue, but I'm not sure if it is enough
to be accepted.

For example, do I need to "winxp,winxp64", here?
http://source.winehq.org/source/dlls/ntdll/version.c?v=wine-1.3.17#L172

TIA,

Ken.