Internalize everything but _Dmain when emitting a -singleobj executable.
This is just a first draft; there are certainly cases
where this still breaks (e.g. naked inline asm).

The general direction looks promising, though: Building a
fully optimized Phobos "Hello World" is noticably faster
with the internalizing enabled (0.7s vs 1s) on my machine.
Hide '-disable-debug-info-verifier'.