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.h b/proc.h
index 03ef044..64c37a9 100644
--- a/proc.h
+++ b/proc.h
@@ -121,7 +121,15 @@
 	void * stack_pointer;      /* To get return addr, args... */
 	void * return_addr;
 	void * arch_ptr;
+
+	/* XXX We would like to replace this with a pointer to ABI
+	 * object that would provide the relevant services, instead of
+	 * checking the necessary flags in the back end ad
+	 * nauseam.  */
 	short e_machine;
+	char e_class;
+
+	/* XXX this shoudl go to ARM's arch_process_data.  */
 #ifdef __arm__
 	int thumb_mode;           /* ARM execution mode: 0: ARM, 1: Thumb */
 #endif