Don't assert when trying to show a Darwin syscall number in an unknown

Don't assert when trying to show a Darwin syscall number in an unknown
syscall class.  This stops Valgrind asserting when doing
--trace-syscalls=yes and a sys_syscall is encountered, in 64-bit mode.
I don't know if this is a correct fix or whether it merely hides
symptoms of logic handling problems for sys_syscall on Darwin.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11203