[OCaml] Accept context explicitly in Llvm_target functions

Llvm_target.intptr_type used to implicitly use global context. As
none of other functions in OCaml bindings do, it is changed to
accept context explicitly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194381 91177308-0d34-0410-b5e6-96231b3b80d8