Renovate CMake files in the `llvm-exegesis` tool.

Renovate CMake files in the `llvm-exegesis` tool.

This attempts to move all tools over to using `add_llvm_library` for
better consistency. After doing this, I noticed it ended up as nearly a
reimplementation of https://reviews.llvm.org/rL342148, which later got
reverted in r342336 (b09a8c9bd9b819741b38071a7ccd95042ef2643a).

With ccache and ninja on a large core machine (40), I haven't run into
build errors, so I'm hopeful it's better now, though it doesn't seem to
be any different / new.

Reviewed By: stephenneuendorffer

Differential Revision: https://reviews.llvm.org/D90970