libsigsegv: disable for ARC

Fixes http://autobuild.buildroot.net/results/5cd/5cd051937a9471f7fb47363e868cb4b61d788d75/build-end.log



The ARC uClibc port doesn't provide ucontext_i.sym file required to enable
context manipulation routines that libsigsegv needs.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>