Amd64 init_secondary() calls initializecpu() while curthread is still

Amd64 init_secondary() calls initializecpu() while curthread is still
not properly set up. r199067 added the call to TUNABLE_INT_FETCH() to
initializecpu() that results in hang because AP are started when kernel
environment is already dynamic and thus needs to acquire mutex, that is
too early in AP start sequence to work.

Extract the code that should be executed only once, because it sets
up global variables, from initializecpu() to initializecpucache(),
and call the later only from hammer_time() executed on BSP. Now,
TUNABLE_INT_FETCH() is done only once at BSP at the early boot stage.

In collaboration with:	Mykola Dzham <freebsd levsha org ua>
Reviewed by:	jhb
Tested by:	ed, battlez