[llvm] Make obj2yaml and yaml2obj LLVM utilities instead of tools

For testing purposes I need a way to build and install FileCheck and
yaml2obj. I had to choose between making FileCheck an LLVM tool and
making obj2yaml and yaml2obj utilities. I think the distinction is
rather arbitrary but my understanding is that tools are things meant for
the toolchain while utilities are more used for things like testing,
which is the case here.

The functional difference is that these tools now end up in the
${LLVM_UTILS_INSTALL_DIR}, which defaults to the ${LLVM_TOOLS_INSTALL_DIR}.
Unless you specified a different value or you added obj2yaml and
yaml2obj to ${LLVM_TOOLCHAIN_TOOLS}, this patch shouldn't change
anything.

Differential revision: https://reviews.llvm.org/D89357