go/ssa: create an 'init' function even if len(files)==0
go/ssa: create an 'init' function even if len(files)==0

Package "unsafe" has no files, and is treated like a regular package since
https://go-review.googlesource.com/37710, and the interpreter needs an
init function with a body for every package.

Change-Id: I27bd7cdb1ecaf01d0b5ed91db57145d1b4f551e1
Reviewed-on: https://go-review.googlesource.com/37711
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,golang/tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools