cmd/link: default to internal linking mode for PIE

[cherrymui]

When using -buildmode=pie, we used to default to internal linking mode. Then we forced external linking because internal linking was found broken. Then we fixed internal linking, but still default to external linking.

As far as I know, internal linking PIE works reasonably well, at least on Linux/AMD64 and Linux/ARM64. We may want to change the default back to internal linking mode.

[gopherbot]

Change https://golang.org/cl/207877 mentions this issue: cmd/link: Revert -buildmode=pie to internal linking

[prattmic]

I saw this after filing #36028. I don't think internal linking should be enabled PIE until that is fixed, unless I'm mistaking something and it's not a common problem.

Regardless, I just sent http://golang.org/cl/210181 to fix that issue.

[gopherbot]

Change https://golang.org/cl/220654 mentions this issue: cmd/link: improve gap detection in TestPIESize