arc: use more universal prefix for default CROSS_COMPILE

As doc/README.ARC says, pre-build ARC toolchains are available at
the Synopsys GitHub page.

The bin files are prefixed with arc(eb)-buildroot-linux- for earlier
releases, but with arc(eb)-snps-linux- for the latest releases.

The symbolic links prefixed with arc(eb)-linux- are also available
for all the release, so those prefixes can be used as the default
CROSS_COMPILE regardless of the toolchains version/variants.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>