[arm] Force a CPU default in the config args defaults

Currently if the user does not specify a default CPU or architecture
the compiler provieds no default values in the spec defaults.  We can
try to work from TARGET_CPU_DEFAULT but pulling that into the driver
is a bit crufty and doesn't really work well with the general
spec-processing model.  A better way is to ensure that with_cpu is
always set appropirately during configure.  To avoid problems with the
multilib fragment processing we defer this until after we have
processed any required fragments before selecting the default.

	* config.gcc (arm*-*-*): Ensure both target_cpu_cname and with_cpu
	are set after handling multilib fragments.  Set target_cpu_default2
	from with_cpu.

From-SVN: r249291

gcc/ChangeLog

 2017-06-16  Richard Earnshaw  <rearnsha@arm.com>
 
+	* config.gcc (arm*-*-*): Ensure both target_cpu_cname and with_cpu
+	are set after handling multilib fragments.  Set target_cpu_default2
+	from with_cpu.
+
+2017-06-16  Richard Earnshaw  <rearnsha@arm.com>
+
 	* config.gcc (arm*-*-fucshia*): Set target_cpu_cname to the real
 	cpu name.
 	(arm*-*-*): Set target_cpu_default2 to a quoted string.

gcc/config.gcc

@@ -3861,6 +3861,8 @@ case "${target}" in
 				TM_MULTILIB_CONFIG="$with_multilib_list"
 			fi
 		fi
+		target_cpu_cname=${target_cpu_cname:-arm6}
+		with_cpu=${with_cpu:-$target_cpu_cname}
 		;;
 
 	fr*-*-*linux*)
@@ -4494,11 +4496,12 @@ case ${target} in
 		;;
 
 	arm*-*-*)
-		if test x$target_cpu_cname = x
+		if test x$with_cpu = x
 		then
-			target_cpu_default2="\\\"arm6\\\""
+			echo "Don't know the target cpu" 1>&2
+			exit 1
 		else
-			target_cpu_default2="\\\"$target_cpu_cname\\\""
+			target_cpu_default2="\\\"$with_cpu\\\""
 		fi
 		;;