[llvm-c] Make LLVMGetTargetFromName actually work

LLVMGetTargetFromName used to compare two char* strings directly.
[OCaml] Fix building of stub libraries
[OCaml] Refactor Llvm_target interface

This commit brings the module structure, argument order and
primitive names in Llvm_target in order with the rest of the bindings,
in preparation for adding TargetMachine API.
[OCaml] Add Target and TargetMachine bindings to Llvm_target