cmd/go: tests failing on linux-amd64-longtest [1.12 backport]

[gopherbot]

@jayconrod requested issue #31946 to be considered for backport to the next 1.12 minor release.

@gopherbot Please open backport issue to 1.12

[jayconrod]

CL 176558 fixes another link error that occurs when -coverpkg covers multiple main packages, seen in #31946 and in #32150. It's a low risk one-line change, and there isn't a good workaround (aside from not using -coverpkg).

CL 164877 and CL 168200 are fixes for closely related problems, and these have already been backported.

[jayconrod]

(Sorry the title is a little misleading. #32150 is the user-reported issue and the motivation for cherry-picking. It was already fixed at tip for a test failure in #31946, which this issue is based on.)

[julieqiu]

Approving since this is a serious problem with no workaround. Please follow the instructions at https://github.com/golang/go/wiki/MinorReleases to create the cherrypick CL.

[gopherbot]

Change https://golang.org/cl/179677 mentions this issue: [release-branch.go1.12] cmd/go: force -coverpkg main packages to be built as libraries

[gopherbot]

Closed by merging 451e6d6 to release-branch.go1.12.