Do not include system call wrappers in libc for old FreeBSD system calls.

The base system libc is only used to run binaries built on FreeBSD 7.0 and
later. It does not need to include system call wrappers for system calls
only used by FreeBSD binaries built on versions older than 7.0. This was
already true for "COMPAT" system calls, but now wrappers for system calls
used on FreeBSD 4 and 6 are excluded as well.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D5597