Fix two places that thought Windows64 is indicated by WIN64 macro.

Fix two places that thought Windows64 is indicated by WIN64 macro.

Everyplace else thinks it's _WIN64, so make these places fall in line.

The pg_regress.c usage is not going to result in any change in behavior,
only suppressing (or not) a compiler warning about downcasting HANDLEs.
So there seems no need for back-patching there.

The libpq/win32.mak usage might represent an actual bug, if anyone were
using this script to build for Windows64, which perhaps nobody is.
Given the lack of field complaints, no back-patch here either.

pg_regress.c problem found by Christian Ullrich, the other by me.