kernel: Make some compile time constant variables const

Those variables are not going to be changed at runtime. Make them const
to avoid potential overwriting. This will also help spotting accidental
global variables shadowing, since the variable's name such as version
is short and commonly used.

This change was inspired by reviewing khng's work D44760.

No functional change intended.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D45227