Add a 'zkill' script, which is more-or-less a fancy (although not necessarily

Add a 'zkill' script, which is more-or-less a fancy (although not necessarily
very robust) version of killall. Because I like making shiny new wheels out of
spare parts.

For use by buildbots when people insist on making cc1 infinite loop. :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86484 91177308-0d34-0410-b5e6-96231b3b80d8