arc: clean-up and move CFLAGS to include/target.mk

Most of currently mentioned CFLAGS in arc770/Makefile
are not really required because:
 [1] "-Os -pipe" are set by default in include/target.mk
 [2] "-fno-caller-saves" gets enabled via menuconfig
     as an extra compiler flag for developers

So the only one that makes sense is "-matomic" and
that one is really essential. Without it many software
packges won't build complainin on unresolved atomic ops.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Jo-Philipp Wich <jow@openwrt.org>
Cc: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 48326