arm: Make init_proc0 static

This function is not used outside of machdep.c and is already static
on arm64 and riscv.

Reviewed by: imp
Effort: CHERI upstreaming
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D54838

(cherry picked from commit 05609c5eff27ea7b382237bd2a937c01c1ceb59a)