align use of cp15_pmccntr_get with its availability

According to ian, the only armv6 cpu we support is the 1176, so this
change is effectively a no-op.
The change is just to make the code more self-consistent.
The issue was noticed by a standalone module build for armv6.

Reviewed by:	ian
MFC after:	3 weeks

sys/arm/include/cpu.h

@@ -20,7 +20,7 @@ void	swi_vm(void *);
 static __inline uint64_t
 get_cyclecount(void)
 {
-#if __ARM_ARCH >= 6
+#if __ARM_ARCH > 6 || (__ARM_ARCH == 6 && defined(CPU_ARM1176))
 #if (__ARM_ARCH > 6) && defined(DEV_PMU)
 	if (pmu_attched) {
 		u_int cpu;