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 <gri@golang.org>

go/ssa/builder.go

@@ -2263,10 +2263,6 @@ func (p *Package) build() {
 	if p.info == nil {
 		return // synthetic package, e.g. "testmain"
 	}
-	if p.files == nil {
-		p.info = nil
-		return // package loaded from export data
-	}
 
 	// Ensure we have runtime type info for all exported members.
 	// TODO(adonovan): ideally belongs in memberFromObject, but