[ARC] Add multilib support for linux targets

[ARC] Add multilib support for linux targets

We used to build baremetal (AKA Elf32) multilibbed toolchains for years
now but never made that for Linux targets since there were problems with
uClibc n multilib setup. Now with help of Crosstool-NG it is finally
possible to create uClibc-based multilibbed toolchains and so we add
relevant CPUs for multilib in case of configuration for "arc*-*-linux*".

This will be essentially useful for glibc-based multilibbbed toolchains
in the future.

gcc/
2018-05-16  Alexey Brodkin <abrodkin@synopsys.com>

        * config.gcc: Add arc/t-multilib-linux to tmake_file for
        arc*-*-linux*.
        * config/arc/t-multilib-linux: Specify MULTILIB_OPTIONS and
        MULTILIB_DIRNAMES

From-SVN: r260434