Regenerate mach_sys.rs to fix struct field alignments

The current 'mach_sys.rs' was generated before rust-bindgen
started translating '#pragma pack(n)' directives into
'#[repr(C, packed(n)]' attributes on the rust struct defintions.
These missing attributes cause debug assertions to fail
because of misaligned access to the fields when serializing
and deserializing the structs for mach_msg calls (see servo#312).

This PR regenerates the 'mach_sys.rs' file using the
latest bindgen and mach headers from XCode MacOS SDK.
It also introduces padding *before* the 'data size' field
for messages with inline data so that they begin at
8-byte aligned addresses as required by rustc.

Signed-off-by: Mukilan Thiyagarajan <[email protected]>