kbuild: Don't remove link-vmlinux temporary files on exit/signal

Keep them around until they are cleaned up by make clean. This
uses a bit more disk space, but makes it easier to debug any
problems with the kernel link process.

Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>