linux(4): Remove function prototypes from the vDSO.

In preparation for vDSO code revision get rid of incomplete vDSO methods
from locore, but leave .note.Linux section commented out.
.note.Linux section is used by glibc rtld to get the kernel version, that
saves one system call call. I'll try to implement it later, if figure out
how to use it with jails.

MFC after: 2 weeks

(cherry picked from commit 815165be201e49d118e93dec6c257cd63b69d158)