bsd-user: Implement do_freebsd_realpathat syscall

Signed-off-by: Mikaël Urankar <mikael.urankar@gmail.com>
Signed-off-by: Karim Taha <kariem.taha2.7@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>