syscall: don't fail if __NR_signalfd is not defined

syscall: don't fail if __NR_signalfd is not defined

lxc fails to build if __NR_signalfd is not defined since version 4.0.0
and
https://github.com/lxc/lxc/commit/bed09c9cc0bec7bbd2442fcce4a2a0f03994cb09

However, some architectures don't define __NR_signalfd but only
__NR_signalfd4. This is the case for example for nios2 or csky:
https://github.com/bminor/glibc/blob/f9ac84f92f151e07586c55e14ed628d493a5929d/sysdeps/unix/sysv/linux/nios2/arch-syscall.h
https://github.com/bminor/glibc/blob/f9ac84f92f151e07586c55e14ed628d493a5929d/sysdeps/unix/sysv/linux/csky/arch-syscall.h

Fixes:
 - http://autobuild.buildroot.org/results/75096a48d2dbda57459523db3ed0952e63f93535

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>