Track elf class in struct Process

Technically only s390 needs this at the moment, but it seems general enough
that it should be alongside the e_machine flag.  Eventually it is desirable
to replace both fields with either a backend-provided ABI enum, or with
an ABI object that would implement the interesting back end calls.
diff --git a/proc.c b/proc.c
index b280df8..bf26180 100644
--- a/proc.c
+++ b/proc.c
@@ -270,6 +270,7 @@
 
 	retp->tracesysgood = proc->tracesysgood;
 	retp->e_machine = proc->e_machine;
+	retp->e_class = proc->e_class;
 
 	/* For non-leader processes, that's all we need to do.  */
 	if (retp->leader != retp)