cmd/compile/internal/gc: add dropped line correction again

The line correction when reporting a missing package clause
was removed before since it wasn't clear that it was needed.
Added it again because of issue 13267.

No explicit test case has been added to test/fixedbugs because
it would require a file that contains a single byte and such a
file doesn't fit the existing test harness. Instead documented
the problematic line in the parser for future reference.

Fixes #13267.

Change-Id: I590fe8f358042aab73acf16c2ed9567872b174f4
Reviewed-on: https://go-review.googlesource.com/16975Reviewed-by: Chris Manghane <cmang@golang.org>

src/cmd/compile/internal/gc/parser.go

@@ -307,6 +307,7 @@ func (p *parser) package_() {
 		mkpackage(p.sym().Name)
 		p.want(';')
 	} else {
+		prevlineno = lineno // see issue #13267
 		p.syntax_error("package statement must be first")
 		errorexit()
 	}