cmd/link: go 1.16 plugin does not initialize global variables correctly when not used directly [1.16 backport]

[gopherbot]

@cherrymui requested issue #44956 to be considered for backport to the next 1.16 minor release.

@gopherbot please backport this to Go 1.16. This is a mis-compilation bug in Go 1.16. Thanks.

[gopherbot]

Change https://golang.org/cl/302449 mentions this issue: [release-branch.go1.16] cmd/compile, cmd/link: dynamically export writable static tmps

[toothrot]

Approved. This is a serious issue with no workaround.

[gopherbot]

Closed by merging ac59d7a to release-branch.go1.16.