libsigsegv: disable for xtensa

The xtensa uClibc port does not provide the ucontext.h context manipulation
routines that libsigsegv needs.

Fixes:
http://autobuild.buildroot.net/results/06d/06d091f2ea8d08a900dd7304bab5eeeeb85f7353/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>