libc: use sys_realpathat directly in realpath

We don't need to use an interposable symbol for this purpose and it's
simpler to just call the syscall in libsys. This resolves a bug where
we were incorrectly using __realpathat in libc not libsys.

While here, drop support for running on a FreeBSD 12 kernel and simplify
includes.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D49048