sigaltstack02: avoid using MINSIGSTKSZ defined by glibc

The value of MINSIGSTKSZ defined by glibc can be different
as the value defined by kernel. Recent change for ppc
is now causing sigaltstack02 to fail:
https://sourceware.org/ml/libc-alpha/2013-08/msg00093.html

Use value which is low enough for all kernel versions.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>