Remove support for IRIX regarding sigaltstack. * m4/sigaltstack.m4 (SV_SIGALTSTACK): Assume that ss_sp is the start address, not the end address, of the alternate stack area. * lib/sigsegv.c: Remove code for IRIX. (stackoverflow_install_handler): Ignore SIGALTSTACK_SS_REVERSED. * lib/sigsegv.in.h: Remove code for IRIX.