[Libomptarget] Fix linking to LLVM dylib (#86397)

Use `LINK_COMPONENTS` parameter of `add_llvm_library` rather than
passing LLVM components directly to `target_link_libraries`, in order to
ensure that LLVM dylib is linked correctly when used. Otherwise, CMake
insists on linking to static libraries that aren't present on
distributions doing pure dylib installs, such as Gentoo.

This fixes a regression introduced
in dcbddc25.