cmd/internal/ld: prevent creation of .dynamic and .dynsym symbols when externally linking
This allows the removal of a fudge in data.go.
We have to defer the calls to adddynlib on non-Darwin until after we have
decided whether we are externally or internally linking. The Macho/ELF
separation could do with some cleaning up, but: code freeze.
Fixing this once rather than per-arch is what inspired the previous CLs.