Delete the msyscall mechanism entirely, since mimmutable+pinsyscalls has

Delete the msyscall mechanism entirely, since mimmutable+pinsyscalls has
replaced it with a more strict mechanism, which happens to be lockless O(1)
rather than micro-lock O(1)+O(log N).  Also nop-out the sys_msyscall(2) guts,
but leave the syscall around for a bit longer so that people can build through
it, since ld.so(1) still wants to call it.