[OCaml] Respect CMAKE_C_FLAGS for OCaml C files

Pass the values of CMAKE_C_FLAGS and CMAKE_C_FLAGS_${CMAKE_BUILD_TYPE}
as -ccopt to ocamlc. This enforces the specific flags used for the LLVM
build to be used for OCaml bindings as well, notably -O and -march
flags.

This also solves the issue of the user being unable to force specific
flags for OCaml bindings builds. Gentoo needs this to enforce -DNDEBUG
consistently between the LLVM build and the split OCaml bindings build.

Differential Revision: https://reviews.llvm.org/D35898

llvm-svn: 309320