[OCaml] Dynamically link LLVM on --enable-shared builds

[OCaml] Dynamically link LLVM on --enable-shared builds

This commit significantly speeds up both bytecode and native
builds of LLVM clients (from ~20 second to sub-second link time),
and allows to invoke LLVM functions from OCaml toplevel.

The behavior for --disable-shared builds is unchanged.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194509 91177308-0d34-0410-b5e6-96231b3b80d8