Set OS versions to target system versions for CROSS_SYSROOT.

When cross-building a package via a CROSS_SYSROOT, set OSVERSION,
_OSRELEASE, and OSREL to the target OS version. OSVERSION is derived
from the <sys/param.h> header in the sysroot. _OSRELEASE cannot be
easily derived from a sysroot as it is a property of a kernel, so
instead generate a "fake" _OSRELEASE from the OSVERSION, but use a
"-CROSS" branch name that will hopefully fail in any places that make
assumptions about branch names.

Approved by: portmgr (bapt)
Differential Revision: https://reviews.freebsd.org/D15812