Create a custom RFPTarget for our patches

At a certain point between Firefox 115 and 128, RFPTarget::Unknown has been removed, but it's compulsory to provide one.

One approach is to find the closest existing target, but sometimes there isn't anyone.

So, @henry suggested we created a RFPTarget for us.

We should think about them because there are some possible caveats:

• we're very close to the 64-bit limit for the targets
• Mozilla might use the value we initially use for our targets (so, should we provide a migration strategy in case?)