Move xmalloc to trivia/util.h We want to use the xmalloc helper throughout the code, not only in the core lib. Move its definition to trivia/util.h and use fprintf+exit instead of say/panic in order not to create circular dependencies.
build: fix build with glibc-2.34 Macros SIGSTKSZ used to be an integral constant but in glibc-2.34 it turned into a runtime function so it cannot be used as constant known size for arrays anymore.
Beyond this, SIGSTKSZ is not enough for alt. signal stack size when you use ASAN, so the size was increased.
