buildlink3, mk/*.builtin.mk: Use TOOLS_CROSS_DESTDIR for file checks.

These are always about the target system, which during cross builds,
or native builds of cross-libtool-base, is relative to
TOOLS_CROSS_DESTDIR.

This is necessary for cross-libtool-base -- the one special package
that is built as a native package as if it were cross-compiled, so
_CROSS_DESTDIR is empty but TOOLS_CROSS_DESTDIR is the cross destdir
-- because cross-libtool-base uses buildlink3 and dlopen.builtin.mk.

No change to native builds because _CROSS_DESTDIR and
TOOLS_CROSS_DESTDIR are both empty in native builds.

XXX Perhaps almost every use of _CROSS_DESTDIR outside mk/pkgformat
should be replaced by TOOLS_CROSS_DESTDIR.

(riastradh)