doc: Remove support for IRIX. * doc/gnulib.texi (Header files): Update. * doc/posix-functions/sig2str.texi: Don't mention IRIX problem. * doc/posix-functions/str2sig.texi: Likewise. * doc/posix-functions/strerror.texi: Likewise. * doc/posix-headers/sys_stat.texi: Don't mention IRIX. * doc/posix-headers/sys_types.texi: Likewise.