unsound transmute in bpf_loader::syscalls

transmute converts between types in unsafe code by reinterpretting the bytes in Rust and forgets the original reference. It effectively disables Rusts built-in type checker by design. While as converts to things smartly, such as float to int, transmute is very dumb about it. Because transmute bypasses built-in type checks, it must be sound. Otherwise, major security issues can occur. Violating soundness can lead to undefined behavior. It has a special section about "transmutation between pointers and integers". In particular, special care must be taken when transmutting between pointers and integers. Agave, the original Solana validator written in Rust, uses transmute in an insecure way. It converts between an integer to a &mut T. This causes the reference to obtain the provenance(space) of an integer, which is none. If T isn't zero-sized, this instantly incurs undefined behavior as a result. Overall, I learned something new about Rust type-safety. Good issue!