[AArch64] Add --with-tune configure flag

[Pop, Sebastian via Gcc-patches]

Hi,

the attached patch 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 the generic 
tuning.

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

Tested on aarch64-linux with bootstrap and regression test.
Ok to commit to trunk and back-port to active branches?

Thanks,
Sebastian

0001-AArch64-add-with-tune-configure-flag.patch
Description: 0001-AArch64-add-with-tune-configure-flag.patch