[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333120 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed