Add offsetof() function

Alongside `sizeof()` and `alignment()` would it be possible to add `offsetof(struct, member)` that works in cross using the crazy magic?

Mesa needs to know these values for a struct it generates but currently has custom C code to determine them, so can't be used in cross builds.

Start by locating the existing sizeof() and alignment() functionality and the cross-compilation logic it uses. Determine how offsetof(struct, member) should expose offsets for generated structs, then verify that the result works for Mesa-style cross builds without requiring custom C code.

Written by the indexing model from the issue text.