[clang] [clang-offload-bundler] Fix finding installed llvm-objcopy

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.

Fixes r369955.

Differential Revision: https://reviews.llvm.org/D68931

llvm-svn: 374754