Use the TSLOG framework to record entry/exit timestamps for hammer_time.
The entry must be logged "manually" using TSRAW rather than TSENTER
since PCPU data structures have not yet been initialized and thus
curthread cannot be accessed; &thread0 is what will become curthread
later in hammer_time.

Other MD initialization code should be similarly instrumented in order
to gain visibility into the time spent before entering mi_startup; this
will require some care and testing from people with access to such
hardware.