Fix struct padding issues with SysV struct classification.

Fix struct padding issues with SysV struct classification.

This change revises the handling of padding bytes between struct fields
and bytes that trail the last field in a struct. These changes address
problems with structs that contain > 8 bytes between fields as well as
C# fixed arrays (which are emitted as a struct with a single field of
the fixed array element type and an explicit size derived from the
number of elements and the size of the element type).

The classification given to a particular byte that is not part of a
formal field depends on whether that byte is between fields or
trails the final field in a struct. Bytes in the former category--
padding bytes--are classified as NO_CLASS as per the SysV ABI spec.
Bytes in the latter category--"leftover" bytes after the final
field of a struct--are assigned the classification of the final
field in the struct. This allows C# fixed-size arrays to work
properly, as their representative structs are logically (but
unfortunately not physically) structs with element-count fields of
the element type. Were they explicit, each of these fields would
have the same classification.