mkDerivation: Don't pass buildInputs to stdenv builder in nativeBuildInputs

When not cross compiling, nativeBuildInputs and buildInputs have
identical behaviour. Currently that is implemented by having
mkDerivation do a concatenation of those variables in Nix code and pass
that to the builder via the nativeBuildInputs attribute.

However, that has some annoying side effects, like `foo.buildInputs`
evaluating to `[ ]` even if buildInputs were specified in the nix
expression for foo.

Instead, pass buildInputs and nativeBuildInputs in separate variables as
usual, and move the logic of cross compilation vs. native compilation to
the stdenv builder script. This is probably a tiny bit uglier but
fixes the previous problem.

Issue #4855.