configure.ac: apply fno-strict-aliasing to entire code base

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
vm.hpp: remove nonsensical pointer truncation

Why the hell would this ever be a sane idea? If our guest isn't mapped into the
lower 4GB of memory, then this will effectively just cause our guest to crash
any time it does a memory write.

Note: After this change I'm finally able to build functional 64-bit SheepShaver
binaries.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>