misc build fixes for compiling with clang

There are still build issues on Mac OSX, but they're now limited to the win32 api calls in `x64_code_cache.cc` (e.g. `RtlDeleteGrowableFunctionTable`) and the inclusion of `DirectXPackedVector.h` in `ivm_intcode.cc`.

Since I know absolutely nothing about this project, I'd review these carefully ;)

@benvanik I'm looking for a project like this to tinker with, but it's all extremely foreign to me. Is there any obvious way to resolve the remaining issues so I can at least get it compiled in order to wrap my head around it more and find something to work on? Here's my latest build output https://gist.github.com/inolen/b5711df3c9147723a228