The function should accept feature strings that old LLVM might not
support.

Simplify the code using the same approach used by
LLVMRustPrintTargetFeatures.

Dummify the function for non 4.0 LLVM and update the tests accordingly.

This commit is an implementation of the new compiler flags required by [RFC
1361][rfc]. This specifically adds a new `cfg` option to the `--print` flag to
the compiler. This new directive will print the defined `#[cfg]` directives by
the compiler for the target in question.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1361-cargo-cfg-dependencies.md