[OCaml] Build stub OCaml libraries for all configured targets

This allows to only link in the needed targets, reducing binary
size and more importantly link time.

Note that this is an incomplete implementation: currently,
LLVM does not have the plumbing which would allow to conditionally
link in AsmPrinter, AsmParser and Disassembler for the targets
which support them. This should be improved in the future.

llvm-svn: 194670