Init functions are problematic because we want cmd/link to be
able to insert an import of runtime/cgo for external linking.
For all the other systems that just means putting runtime/cgo into
the binary. The linker is not set up to generate calls to init functions,
and luckily this one can be avoided entirely.

This means people don't have to import _ "runtime/cgo" in their
iOS programs anymore. The linker's default import is now enough.

This CL also adjusts cmd/go to record the linker's default import,
now that the explicit import is gone.

Change-Id: I81d23476663e03664f90d531c24db2e4f2e6c66b
Reviewed-on: https://go-review.googlesource.com/68490
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>