tools/golang/xenlight: Rework gengotypes.py and generation of *.gen.go

tools/golang/xenlight: Rework gengotypes.py and generation of *.gen.go

gengotypes.py creates both "types.gen.go" and "helpers.gen.go", but
make can start gengotypes.py twice. Rework the rules so that
gengotypes.py is executed only once.

Also, add the ability to provide a path to tell gengotypes.py where to
put the files. This doesn't matter yet but it will when for example
the script will be run from tools/ to generate the targets.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>