arch/ctx.h: Update ctx members declaration order

arch/ctx.h: Update ctx members declaration order

Out-of-order designated initialization is not allowed in C++, so change
the `ctx` declaration order to match the `ukarch_ctx` definition order.

Signed-off-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #1085