Mach: Fvmlib is packed and implements Pwrite

[m4b]

warning: borrow of packed field is unsafe and requires unsafe function or block (error E0133)                                                         
   --> /home/m4b/projects/goblin/src/mach/load_command.rs:145:37                                                                                      
    |                                                                                                                                                 
145 | #[derive(Debug, Clone, Copy, Pread, Pwrite, IOread, IOwrite, SizeWith)]                                                                         
    |                                     ^^^^^^                                                                                                      
    |                                                                                                                                                 
    = note: #[warn(safe_packed_borrows)] on by default                                                                                                
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!                 
    = note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>                                                   
    = note: fields of packed structs might be misaligned: dereferencing a misaligned pointer or even just creating a misaligned reference is undefined behavior

I guess simplest solution is to remove Pwrite impl (luckily it's a deprecated struct, so likely won't ever be needed by any writer); nevertheless, need to figure out what to do here, though it probably belongs in scroll...

[philipc]

Why does it need to be packed instead of repr(C)? The definition in LLVM is a plain C struct.