Log Message:
*/builtin.mk: Use ${_CROSS_DESTDIR:U} for build-time file checks.

These are questions about the target system, whose files at
build-time are all relative to ${_CROSS_DESTDIR} if it is defined,
i.e., if USE_CROSS_COMPILE is set to yes.

No change to native builds because ${_CROSS_DESTDIR:U} is empty in
them.  (Possible minor change by adding :Q to ${H_FOO} in command
lines, but if this makes a difference it likely fixes problems.)