cmd/gc: preserve safe annotation of package def.

A package file may begin as either "package foo" or
"package foo safe". The latter is relevant when using -u.
https://codereview.appspot.com/6903059 resulted in the distinction
being dropped when a package was read for the second or later time.
This CL records whether that "safe" tag was present,
and includes it in the dummy statement generated for the lexer.

R=golang-dev, r, minux.ma, daniel.morsing, iant
CC=golang-dev
https://codereview.appspot.com/8255044