gcc: arc-2017.03-rc2: Fix "unrecognized supposed constant" error

This fixes the following problem:
------------------------------------>8--------------------------------
arc-linux-gcc -c -Os -fPIC iso9660.i
iso9660.c: In function 'strip_trail':
iso9660.c:155:1: error: unrecognized supposed constant
 }
 ^
(unspec:SI [
        (symbol_ref:SI ("*.LANCHOR1") [flags 0x182])
    ] ARC_UNSPEC_GOTOFFPC)
iso9660.c:155:1: internal compiler error: in arc_legitimate_constant_p, at config/arc/arc.c:6028
------------------------------------>8--------------------------------

Found by Buildroot autobuilder [1].

The fix [2] is in arc-2017.03 development branch of ARC GCC and once it
becomes a part the next release of ARC tools this should be removed
from Buildroot.

[1] http://autobuild.buildroot.net/results/c90/c909e8c397ab972b6aa4d370572cad4fae284d00/build-end.log
[2] https://github.com/foss-for-synopsys-dwc-arc-processors/gcc/commit/139fed9d29ab935b3bc5159c0bdf7b8b8a39442d



Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>