[llvm] Disable linking llvm-exegesis to dylib

Force linking llvm-exegesis to static LLVM libraries instead of dylib
to prevent duplicate symbols due to linking both.  Ideally, we'd want
to link to the dylib only here but the target sub-libraries use hidden
symbols from LLVM target libraries and therefore linking the dylib
fails.

Differential Revision: https://reviews.llvm.org/D81922