kbuild: remove the target in signal traps when interrupted

When receiving some signal, GNU Make automatically deletes the target if
it has already been changed by the interrupted recipe.

If the target is possibly incomplete due to interruption, it must be
deleted so that it will be remade from scratch on the next run of make.
Otherwise, the target would remain corrupted permanently because its
timestamp had already been updated.

Thanks to this behavior of Make, you can stop the build any time by
pressing Ctrl-C, and just run 'make' to resume it.

Kbuild also relies on this feature, but it is equivalently important
for any build systems that make decisions based on timestamps (if you
want to support Ctrl-C reliably).

However, this does not always work as claimed; Make immediately dies
with Ctrl-C if its stderr goes into a pipe.

  [Test Makefile]

    foo:
            echo hello > $@
            sleep 3
            echo world >> $@

  [Test Result]

    $ make                         # hit Ctrl-C
    ec...