libsys: Don't create or expose __realpathat

Previously, __realpathat was in libc and libsys (as is currently
standard), but not exported from libc which meant the stub in libc was
not filtered and thus libc's copy of the syscall was used. This broke
an upcoming change to CheriBSD limiting syscalls to libsys.

The realpath(3) implementation now uses sys_realpathat so there are no
consumers of realpathat. Switch it to PSEUDO (only _foo and sys_foo
symbols) and remove __realpathat from Symbol.map.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D49049