Get rid of netbsd_lchown and netbsd_msync syscall entries.

No valid FreeBSD binary very called them (they would call lchown and
msync directly) and we haven't supported NetBSD binaries in ages.

This is a respin of r335983 with a workaround for the ancient BFD linker
in the libc stubs.

Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16193