cmd/link: incorrect GC bitmap when global's type is in another shared object [1.14 backport]

[gopherbot]

@aclements requested issue #39927 to be considered for backport to the next 1.14 minor release.

@gopherbot, please backport to Go 1.14 and 1.15.

[dmitshur]

@aclements Can you please include a short rationale about why the backport might be needed? (Per MinorReleases.) Thanks.

[gopherbot]

Change https://golang.org/cl/240511 mentions this issue: [release-branch.go1.14] cmd/link: fix GC data reading from shared library (attempt 2)

[aclements]

Rationale for backport: This causes the garbage collector to incorrectly free reachable objects when using shared build mode, leading to memory corruption and crashes. There is no workaround (other than not using shared build mode).

[dmitshur]

Backport approved for Go 1.14 (this issue) and 1.13 (#39956) based on that rationale.

[gopherbot]

Change https://golang.org/cl/241978 mentions this issue: [release-branch.go1.13] cmd/link: fix GC data reading from shared library (attempt 2)