makesyscalls.lua: improve generated file style(9) compliance

We generally like to avoid style changes when other changes are not
planned. In this case there are some makesyscalls.lua changes in the
pipeline, and this cleans up style nits in generated files that were
highlighted by experiments with clang-format.

Reviewed by: brooks, kevans
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30235