The fchmodat2 syscall on Linux is effectively the same as fchmodat
on FreeBSD, whereas fchmodat on Linux is fchmodat on FreeBSD, minus
the flags argument.
This change provides the necessary glue in order to call kern_fchmodat
properly via the Linuxulator.
NB: glibc transparently passes through the flags argument for
fchmodat*, so this is [largely] a no-op for end-users, but it affects
those who compiled binaries specifically using the fchmodat2 syscall
--which likely includes newer versions of glibc, etc.
MFC after: 2 weeks