arc770: Remove arc770 target

arc770: Remove arc770 target

The arc700 target is not booting up since some time, see here:
https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/issues/400

It looks like there is a problem in the toolchain when using glibc.

Currently no one is working on fixing this problem, remove the target
instead. This target also does not have many users we are aware of.

If someone wants to have this target back, feel free to add a fixed
version of this target again.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Acked-by: Stijn Tintel <stijn@linux-ipv6.be>