xterm: disable with ARC external toolchain

This package fails when building with ARC external toolchain

Marking this with special comment "# ARC toolchain issue" as the package
is to be enabled as soon as the issue with the ARC toolchain is
resolved.

Fixes:
  http://autobuild.buildroot.net/results/28a/28a92049a6ceef005787c5779f77ecf3fe8ad642//



Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>