main/s390-tools: disable _FORTIFY_SOURCE in zipl

Disabling _FORTIFY_SOURCE fixes SIGFPE in zipl:

Program received signal SIGFPE, Arithmetic exception.
memcpy (__n=7976, __os=<optimized out>, __od=0x3fffdff60c0) at /usr/include/fortify/string.h:53
53              return __builtin_memcpy(__od, __os, __n);

disassebly shows that the exception happens on the jump instruction to
memcpy. Work around it by disabling _FORTIFY_SOURCE for zipl.

(cherry picked from commit 9fa93539)