counter(9): unbreak amd64 following r336020

Apply temporary fix to counter until daylight hours.
The fact that the assembly for counter_u64_add relied on the sizeof(struct pcpu) was
the basis for the otherwise arbitrary offset never came up in D15933.
critical_{enter,exit} is now inline so the only real added overhead is the
added (mostly false) conditional branch in exit.

sys/amd64/include/counter.h

@@ -83,11 +83,18 @@ counter_u64_zero_inline(counter_u64_t c)
 static inline void
 counter_u64_add(counter_u64_t c, int64_t inc)
 {
-
+	int64_t *p;
+#ifdef notyet
 	__asm __volatile("addq\t%1,%%gs:(%0)"
 	    :
 	    : "r" ((char *)c - (char *)&__pcpu[0]), "ri" (inc)
 	    : "memory", "cc");
+#endif
+	/* temporary */
+	critical_enter();
+	p = zpcpu_get(c);
+	*p += inc;
+	critical_exit();
 }
 
 #endif	/* ! __MACHINE_COUNTER_H__ */