MSI: Switch to 64bit MSI Packaging

* src/Makefile.am, src/gpg4win.mk.in (*.wixlib): Add
-arch x64 to candle calls.
* src/make-msi.pl: Update for Win64.

64 bit packaging gives us the choice in which registry
to write. This was required because the 32 bit package
did not allow this. To still have 64 bit GpgOL and GpgEX
support in the previous package we had to use a hack
to call "DLLInstallServer" in both plugins. Now we
also don't need that anymore.