bsd.prefs.mk: Set _CROSSDIR_SUFFIX under TOOLS_CROSS_COMPILE too.

This way, cross-libtool also gets a per-cross-platform workdir, so
you can share the same tree between multiple cross builds.

No change to native builds because the difference between
TOOLS_USE_CROSS_COMPILE and USE_CROSS_COMPILE only affects special
packages related to cross-builds, namely cross-libtool.

(riastradh)