build: don't add -fno-plt for ARC

Curent ARC toolchain fails to build libstdc++ if -fno-plt is used.
Lots of following error messages appear:
------------------->8------------------
...
staging_dir/toolchain-arc_arc700_gcc-arc-2015.06_uClibc-1.0.9/arc-openwrt-linux-uclibc/bin/ld:
BFD (GNU Binutils) 2.23.2 assertion fail elf32-arc.c:2786
collect2: error: ld returned 1 exit status
------------------->8------------------

In newer binutils (still in development) for ARC rewritten from
scratch this seem to not happen, so once new binutils for ARC hit
the street this patch might be reverted.

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: 48642