Add Win64 calling convention.
Use the correct calling convention for extern rust functions.
Don't use win64 calling convention on 32-bit machines.
auto merge of #10527 : eholk/rust/win64, r=alexcrichton

This was needed to access UEFI boot services in my new Boot2Rust experiment.

I also realized that Rust functions declared as extern always use the C calling convention regardless of how they were declared, so this pull request fixes that as well.