Internal Linking Support

[gdams]

This module doesn't support building with internal linking. I've skipped TestBoringInternalLink for now

[qmuntal]

I'll be submitting a series of upstream CLs to fix this issue in the Go linker itself.

[qmuntal]

643279: cmd/link: set SDYNIMPORT for host objects with empty library path | https://go-review.googlesource.com/c/go/+/643279

[qmuntal]

645235: cmd/link: improve support for dynamic import variables on Darwin | https://go-review.googlesource.com/c/go/+/645235