runtime: GOTRACEBACK=crash doesn't print user stack for threads on system stack (backport)

[aclements]

https://golang.org/cl/38012 fixes #19494.

[gopherbot]

CL https://golang.org/cl/39600 mentions this issue.

[bradfitz]

Is this worthy of a backport? It doesn't appear to be a regression from Go 1.7? At least #19494 doesn't say so.

Ask @rsc if he's sitting there.

[aclements]

It's not a regression, but it has interfered with debugging runtime issues in 1.8. The fix is trivial and obviously correct (though the test is mighty hairy).

[aclements]

Cherry-picked to release.