bsd-user: OS-agnostic 64-bit SYSCTL types

Use existence of type as #ifdef condition rather than FreeBSD-specific
version check, as suggested by Patrick Welche.

Also handle the signed (CTLTYPE_S64) case identically to the unsigned
(CTLTYPE_U64) case, per later patches in the FreeBSD ports tree
(emulators/qemu-devel/files/patch-z-arm-bsd-user-001).

Signed-off-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
