Add bsd-user/freebsd/make_syscall_nr_h.sh to generate syscall_nr.h.

make_syscall_nr.h.sh is added so syscall_nr.h can be generated from
<sys/syscall.h>.  Maybe this should be configure or one of the
makefiles. Also report "qemu: unsupported syscall: #" via gemu_log
when we try to emulate an unsupported system call.