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.

sys/amd64/amd64/machdep.c

@@ -1525,6 +1525,8 @@ hammer_time(u_int64_t modulep, u_int64_t physfree)
 	size_t kstack0_sz;
 	int late_console;
 
+	TSRAW(&thread0, TS_ENTER, __func__, NULL);
+
 	/*
  	 * This may be done better later if it gets more high level
  	 * components in it. If so just link td->td_proc here.
@@ -1774,6 +1776,8 @@ hammer_time(u_int64_t modulep, u_int64_t physfree)
 #endif
 	thread0.td_critnest = 0;
 
+	TSEXIT();
+
 	/* Location of kernel stack for locore */
 	return ((u_int64_t)thread0.td_pcb);
 }