[llvm-objcopy] Improve tool selection logic to recognize llvm-strip-$major as strip

Debian and some other distributions install llvm-strip as llvm-strip-$major (e.g. `/usr/bin/llvm-strip-9`)

D54193 made it work with llvm-strip-$major but did not add a test.
The behavior was regressed by D69146.

Fixes https://github.com/ClangBuiltLinux/linux/issues/940

Reviewed By: alexshap

Differential Revision: https://reviews.llvm.org/D76562