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.
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.-]\+//'`