bsd-user: move strace OS/arch dependent code to host/arch dirs

This change moves host OS and arch dependent code for the sysarch
system call related to the -strace functionality into the
appropriate host OS and target arch directories.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Sean Bruno <sburno@FreeBSD.org>
[ imp integrated minor build fixes from sbruno ]
Signed-off-by: Warner Losh <imp@FreeBSD.org>
Message-Id: <20201218205451.10559-4-imp@freebsd.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>