[llvm-strip] Minor fix of the usage of TableGen

This is a small follow-up to the revisions r333117 and r331663.

1. Avoid the name conflicts of the generated variables for prefixes.
2. Apply clang-format -i -style=llvm to llvm-objcopy.cpp once again.
3. Add a test for the flag with double dash.

Test plan: make check-all

llvm-svn: 333120