add --with-tune configure flag

fixes a configure error on Arm64 when passing --with-tune=... to configure:
```
This target does not support --with-tune.
Valid --with options are: abi cpu arch
```
The missing flag sets target tuning to a different value than generic tuning.

gcc/
	* config.gcc: Add --with-tune to AArch64 configure flags.

gcc/config.gcc

@@ -4083,9 +4083,8 @@ fi
 supported_defaults=
 case "${target}" in
 	aarch64*-*-*)
-		supported_defaults="abi cpu arch"
-		for which in cpu arch; do
-
+		supported_defaults="abi cpu arch tune"
+		for which in cpu arch tune; do
 			eval "val=\$with_$which"
 			base_val=`echo $val | sed -e 's/\+.*//'`
 			ext_val=`echo $val | sed -e 's/[a-z0-9.-]\+//'`