cmd/cgo: alignment issue with int128 inside of a struct [1.23 backport]

[gopherbot]

@ianlancetaylor requested issue #69086 to be considered for backport to the next 1.23 minor release.

@gopherbot Please backport this issue.

This never worked so it is not a regression. But it seems worth fixing because without it it's quite hard to use a C struct with an int128 field.

[dmitshur]

@ianlancetaylor This backport is approved, please send backport CLs when you have a chance. Thanks.

[gopherbot]

Change https://go.dev/cl/611296 mentions this issue: [release-branch.go1.23] cmd/cgo: correct padding required by alignment

[gopherbot]

Closed by merging CL 611296 (commit 80ff7cd) to release-branch.go1.23.