plugin: loading multiple plugins corrupts stdlib types in Go 1.27 [1.27 backport]

[gopherbot]

@cherrymui requested issue #81303 to be considered for backport to the next 1.27 minor release.

@gopherbot please backport this to Go 1.27. This is a regression in Go 1.27, causing real programs panic at run time. Thanks.

[liggitt]

cc @dims

[gopherbot]

Change https://go.dev/cl/827664 mentions this issue: [release-branch.go1.27] runtime: don't add the itabs of a plugin twice