cmd/cgo: generates code with inappropriate array copy [1.12 backport]

[gopherbot]

@ianlancetaylor requested issue #32579 to be considered for backport to the next 1.12 minor release.

@gopherbot please open backport to 1.12

[gopherbot]

Change https://golang.org/cl/183627 mentions this issue: [release-branch.go1.12] cmd/cgo: fix inappropriate array copy

[gopherbot]

Closed by merging 77c14d2 to release-branch.go1.12.