For some reason, clang 3.3 optimizes out too much of MemoryBits::get(), making
it sometimes return 0 when it shouldn't. This happens even when only
type-punning through unions and void pointers (which should be well-defined),
and only when get() is inlined into its caller. We might be unwittingly
invoking some undefined behaviour somewhere, or it might be a clang/llvm
optimizer bug.

This cuts the object file size (for both execution.cpp and constants.cpp)
roughly in half and could help with compile times.

Currently runs global constructors (priority ordering and destructors to be
done) and calls exit() with main's return value.