Use a (signed) int32_t counter instead of an `unsigned int' counter

for the GPROF4 case.  This allows a simpler method to be used for
non-statistical profiling (it allows overhead adjustments to be
subtracted from one counter without harm if that counter goes
negative; otherwise the adjustment would have to be distributed).

32 bit counters were already too small for GPROF4 with a 200MHz
clock.  int64_t counters should be used.

usr.bin/gprof/gprof.h

@@ -82,7 +82,7 @@ typedef int	bool;
 long	hz;
 
 #ifdef GPROF4
-typedef	unsigned int UNIT;
+typedef	int32_t UNIT;
 #else
 typedef	u_short UNIT;		/* unit of profiling */
 #endif