Build an MSI installer for the new Win64 binaries.

Build an MSI installer for the new Win64 binaries.

The MSI format has a fixed field for target architecture, so there's
no way to build a single MSI that can decide at install time whether
to install 32-bit or 64-bit (or both). The best you can do along those
lines, apparently, is to have two MSI files plus a bootstrap .EXE that
decides which of them to run, and as far as I'm concerned that would
just reintroduce all the same risks and annoyances that made us want
to migrate away from .EXE installers anyway.