runtime: fixes for arm64 shared libraries

runtime: fixes for arm64 shared libraries

Building for shared libraries requires that all functions that are declared
have an implementation and vice versa so make that so on arm64.

It would be nicer to not require the stub sigreturn (it will never be called)
but that seems a bit awkward.

Change-Id: I3cec81697161b452af81fa35939f748bd1acf7fd
Reviewed-on: https://go-review.googlesource.com/13995
Reviewed-by: David Crawshaw <crawshaw@golang.org>