arm timer fix

. set 'done' once initialized so 32-bit read frc works,
	  thanks to keesj
	. make sure the software-implemented upper 32 bit of the 64-bit
	  "tsc" value works OK by adding an assert in one of its calls

Change-Id: I5ce24fea919f4610c6a86ac7ec9f04b1815620c2

kernel/arch/earm/arch_clock.c

@@ -10,6 +10,8 @@
 #include "kernel/glo.h"
 #include "kernel/profile.h"
 
+#include <assert.h>
+
 
 #include "kernel/spinlock.h"
 
@@ -61,6 +63,7 @@ void context_stop(struct proc * p)
 	u64_t * __tsc_ctr_switch = get_cpulocal_var_ptr(tsc_ctr_switch);
 
 	read_tsc_64(&tsc);
+	assert(tsc >= *__tsc_ctr_switch);
 	tsc_delta = tsc - *__tsc_ctr_switch;
 	p->p_cycles += tsc_delta;