plugin: variable not initialized properly [1.21 backport]

[gopherbot]

@cherrymui requested issue #62430 to be considered for backport to the next 1.21 minor release.

@gopherbot please backport this to Go 1.21. This is a regression that causes valid programs to run incorrectly.

[gopherbot]

Change https://go.dev/cl/526575 mentions this issue: cmd/link: avoid deadcode of global map vars for programs using plugins

[gopherbot]

Closed by merging 556e9c3 to release-branch.go1.21.