make: Add tags target and gitignore the tags file

Add make target to generate ctags tags file and add it to .gitignore.

Signed-off-by: Siddharth Chandrasekaran <sidcha@amazon.de>
Message-Id: <20210610113128.5418-1-sidcha@amazon.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>