Clarify the role of packed types in computing natural alignment

[rjmccall]

AAPCS defines the natural alignment of a composite type as the maximum of the alignment of the members. Both GCC and Clang appear to honor __attribute__((packed)) when computing natural alignment, essentially setting the natural alignment to 1; obligatory godbolt link. This is probably worth clarifying.

[rearnsha]

The AAPCS already says The member alignment of an element of a composite type is the alignment of that member after the application of any language alignment modifiers to that member

which I think covers it.