Define <target-arch> for win32 and win64 #1439

After starting building windows 64-bit builds OpenCPN/OpenCPN#2027 comes back to us again. In short, we don't define the target-arch metadata field in a consistent way.

This is not a practical problem at this point, but the data should be consistent in any case. In the case we start to build aarch64 windows plugins it will be crucial to get it right.

To my understanding, current situation is:

• The core does not currently care about target-arch for windows, be it 32 or 64 bits.
• Shipdriver uses x86 for both 32 and 64 bits plugins, a result of the Plugins: Wrong architecture for win32 plugins. OpenCPN#2027 discussions.
• Testplugin uses x86_64 for both 32 and 64 bits.

The solution seems simple: we should use x86 for 32-bit plugins and x86_64 for 64-bit. This will match other plugins, use the same xsd schemas and avoid bringing anything new into the ecosystem.

Unless there is more input I will file downstream bugs.