[cmake] Don't set install rules for tblgen if building utils is disabled

Summary:
This is a follow up to D64032. Afterwards if building utils is disabled
and cross compilation is attempted, CMake will complain that adding
`install()` directives to targets with EXCLUDE_FROM_ALL set is "undefined".
Indeed, it appears depending on the CMake version and the selected
Generator, the install rule will error because the underlying target isn't
built. Fix that by not adding the install rule if building utils is not
requested. Note that this doesn't prevent building tblgen as a
dependency in not cross-build, even if building tools is disabled.

Reviewed By: smeenai
Differential Revision: https://reviews.llvm.org/D64225

llvm-svn: 366108