go/types, types2: correct alignment of atomic.Int64

go/types, types2: correct alignment of atomic.Int64

atomic.Int64 has special logic in the compiler to ensure it's 8-byte
aligned on 32-bit architectures. The equivalent logic is missing in
go/types, which means the compiler and go/types can come to different
conclusions about the layout of types.

Fix this by mirroring the compiler's logic into go/types.

Fixes #53884.

Change-Id: I3f58a56babb76634839a161ca174c8f085fe3ba4
Reviewed-on: https://go-review.googlesource.com/c/go/+/417555
Reviewed-by: Robert Findley <rfindley@google.com>