Auto merge of #147375 - folkertdev:power-align-union, r=RalfJung

ignore power alignment rule on unions

fixes rust-lang/rust#147348

The power alignment rule only applies to the non-first field of a struct, and so should not apply to unions at all.

The current code also does not consider enums (whose fields might be, morally, structs). Given that C does not actually have ADTs like this it's probably fine from a compatibility perspective, but I'll leave that to the powerpc folks.

cc `@daltenty` `@gilamn5tr`
r? compiler

Author: bors