openbsd: fix sigcontext struct and avoid defining fxsave64
`sc_fpstate` member of `struct sigcontext` is a `struct fxsave64 *`.
use *anyopaque to represent it.
avoid to defining `fxsave64` as it is a packed struct with some arrays.
8bcb962adaSébastien Marie committed on 8/26/2022, 3:30:07 PM· committed by Isaac Freundparent2e0ff6c