ninjatool: rebuild multi-output targets if outputs are missing

ninjatool: rebuild multi-output targets if outputs are missing

The "stamp file trick" used to group targets of a single multi-output rule
prevents the user from deleting one such target in order to force its
rebuild.  Doing so will not touch the stamp file, and therefore only
the dummy ":" command will be executed.

With this patch, ninjatool writes rules that force-rebuild the stamp
file if any of its outputs are missing.  Rebuilding the missing
target therefore causes the stamp file to be rebuilt too.

Reported-by: Markus Armbruster <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>