[build] Fixed build break for OCaml bindings.

After r273302 LLVM_SYSTEM_LIBS may have entries that already have
-l prefix and we need to deal with that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273333 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed