Fail build on non 64-bit targets

As discussed on Slack and during sync meetings, we should make it
explicit that we only support 64bit. While other architectures may be
viable to support, currently we assume that usize == u64 and nobody
expressed interest in 32bit support.

Signed-off-by: Erik Schilling <[email protected]>

Author: Erik Schilling

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Added
 ### Changed
+- [[#275](https://github.com/rust-vmm/vm-memory/pull/275)] Fail builds on non 64-bit platforms.
 ### Fixed
 ### Removed
 ### Deprecated

src/lib.rs

@@ -20,6 +20,10 @@
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
 
+// We only support 64bit. Fail build when attempting to build other targets
+#[cfg(not(target_pointer_width = "64"))]
+compile_error!("vm-memory only supports 64-bit targets!");
+
 #[macro_use]
 pub mod address;
 pub use address::{Address, AddressValue};