makesyscalls.lua: Make more luaish

x["y"] can be written as x.y, which looks better and is a more typical
lua idiom.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D39709

(cherry picked from commit 1dd350fce0aad85c559b962654f71d1449f21727)