Makefile: Fix tag file generation targets

"ctags" produces a file named "tags", not "ctags". It doesn't look
reasonable to use phony target name as a file name to remove. Just use
exact file names to remove in "ctags" and "TAGS" target receipts.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
Message-Id: <1465495115-24665-1-git-send-email-sergey.fedorov@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>