init_main: Use TUNABLE_INT_FETCH to initialize verbose_sysinit

[zlei]

Summary

It is too late to initialize verbose_sysinit via TUNABLE_INT, as
TUNABLE_INT run at the order of SI_SUB_TUNABLES, thus any subsystems
those have order prior or equal to SI_SUB_TUNABLES are not logged.

MFC after: 1 week

Test Plan

Boot with debug.verbose_sysinit enabled.

# dmesg
---<<BOOT>>---
subsystem 6fffff
   madt_register(0)... done.
   apic_init(0)... done.
subsystem 700000
   mp_setmaxid(0)... done.
   tunable_int_init(&__tunable_int_102)... done.
   tunable_int_init(&__tunable_int_104)... done.
   tunable_int_init(&__tunable_int_106)... done.
   tunable_int_init(&__tunable_int_197)... done.
...

[kevans]

Heh, whoops-