OCAMLFLAGS can contain =, don't use = with sed

Like HOST_LDFLAGS, etc. OCAMLFLAGS can contain =, so use ! as the substitution
separator instead of = (otherwise, sed might error).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221879 91177308-0d34-0410-b5e6-96231b3b80d8