[gn build] Add some llvm/tools: llvm-exegesis, llvm-extract, llvm-link
Also add build file for dependency llvm/lib/ExecutionEngine/MCJIT.
The exegesis stuff is pretty hairy and knows a lot about Target internals (in
general, not specifically in the GN build). I put the llvm-tblgen -gen-exegesis
call in llvm/tools/llvm-exegesis/lib/X86, instead of in llvm/lib/Target/X86
where it is in CMake land, and asked on D52932 why it's in that place in the
Needed for check-llvm.
Differential Revision: https://reviews.llvm.org/D56167