[lldbtestsuite] llvm-objcopy is now required to run the lit tests.

There's now a test using llvm-objcopy in lit/.
This doesn't fail on the bot(s) because `llvm-objcopy` is probably
already available there, but if you get a fresh checkout and run
`ninja check-lldb` you'll observe the failure as it's not tracking
the dependency correctly. This fixes the problem on my machine,
and probably everywhere else.

llvm-svn: 326919