Fix ocaml tests for 64-bit MacOS systems. LLVM is currently built

as 32-bit code by default, and if gcc defaults to 64-bit code then ocamlc
requires a -cc "gcc -arch i386" option.  We were hardcoding -cc g++
and throwing away any other compiler options that were determined when
ocamlc was configured and built.

llvm-svn: 76658