Allow finding installed llvm-objcopy in PATH if it's not present
in the directory containing clang-offload-bundler. This is the case
if clang is being built stand-alone, and llvm-objcopy is already
installed while the c-o-b tool is still present in build directory.
This is consistent with how e.g. llvm-symbolizer is found in LLVM.
However, most of similar searches in LLVM and Clang are performed
without special-casing the program directory.