generate: concurrent map write

[mvdan]

About a quarter of the times I run the program, I get a fatal error: concurrent map iteration and map write.

I presume the race has been there for a long time, but the older versions of Go I used in the past didn't notice that.

Not a hugely pressing matter, since this is just a code generator I use, and the output still seems to be stable. But I should still fix this at some point.