bsd.prefs.mk: Unconditionally define TOOLS_CROSS_DESTDIR.

- If we're cross-compiling or building cross-libtool-base, alias for
  CROSS_DESTDIR.

- Otherwise, empty.

This will be convenient for sprinkling TOOLS_CROSS_DESTDIR in various
places throughout mk/ without worrying about disrupting native
builds.

No change to native builds here because all current references are
conditional on USE_CROSS_COMPILE = yes.

(riastradh)