MSI: Add x86 targets

* src/Makefile.am: Add target for generic x86 wixlib.
* src/gpg4win.mk.in: Add templates for x86 targets.
* src/make-msi.pl: Remove arch from package.

To have the architecture dependent ohn the candle call is
suggested by wixtools. This makes it possible to simply
create an x86 package, too. The default build only creates
x64 packages.