[CMake] Restrict libxar linkage to just llvm-objdump

[CMake] Restrict libxar linkage to just llvm-objdump

At some point we're going to need libObject to have this dependency, but as it is now this is causing too many headaches. This commit will reduce the linkage to just llvm-objdump where it is strictly needed, and we'll cross the libObject bridge later when we need it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270866 91177308-0d34-0410-b5e6-96231b3b80d8