Make it an error to build an ARM kernel with COMPAT_FREEBSDn where n < 10.

We changed the ABI for ARM in 10, an removed support for the old ABI in 11,
as such binaries from these releases are unable to be run on a head kernel.

Reviewed by:	bz, emast
Sponsored by:	ABT Systems Ltd
Differential Revision:	https://reviews.freebsd.org/D5652

sys/arm/arm/machdep.c

@@ -178,6 +178,12 @@ DB_SHOW_COMMAND(vtop, db_show_vtop)
 #define	debugf(fmt, args...)
 #endif
 
+#if defined(COMPAT_FREEBSD4) || defined(COMPAT_FREEBSD5) || \
+    defined(COMPAT_FREEBSD6) || defined(COMPAT_FREEBSD7) || \
+    defined(COMPAT_FREEBSD9)
+#error FreeBSD/arm doesn't provide compatibility with releases prior to 10
+#endif
+
 struct pcpu __pcpu[MAXCPU];
 struct pcpu *pcpup = &__pcpu[0];