prefer tsc timecounter when it's good

Guys,

what do you think about the following change?
The idea is mark TSC as the best timecounter when it's invariant and synchronized
between cores.
Unfortunately I don't have code to auto-detect the synchronization and keep
relying on the corresponding tunable.  I thought about auto-setting it for
single-package configurations, but even that information is currently not trivial
to get out of our mp (i386/amd64) machdep code.

--- a/sys/x86/x86/tsc.c
+++ b/sys/x86/x86/tsc.c
_at__at_ -169,6 +169,9 _at__at_ init_TSC_tc(void)
 			printf("TSC timecounter disabled: APM enabled.\n");
 	}

+	if (tsc_is_invariant)
+		tsc_timecounter.tc_quality = 1200;
+
 #ifdef SMP
 	/*
 	 * We can not use the TSC in SMP mode unless the TSCs on all CPUs

-- 
Andriy Gapon