[IA64] cleanup in fsys.S

beautify coding style for zeroing end of fsyscall_table entries.
Remove misleading __NR_syscall_last and add more comments.
Drop (now unneeded) "guard against failure to increase NR_syscalls"

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>